docs: add worked example for withings-export (#40)

Moves the withings-export worked example out of quantcli/common (where
it was misfiled in PR #24) and into this repo where it belongs — next
to the binary it documents. Co-references in GETTING_STARTED.md will
be updated in a follow-up PR against quantcli/common.

Co-authored-by: LeadGoEngineer <noreply@paperclip.ing>

Author: Terastar-Paperclip

docs/example.md

@@ -0,0 +1,214 @@
+# Worked example: withings-export
+
+**Audience:** Withings device owners (scale, sleep tracker, activity watch) who want their health data in a terminal or piped to an LLM agent.
+
+**Prerequisites:** `withings-export` installed (see [GETTING_STARTED.md](https://github.com/quantcli/common/blob/main/GETTING_STARTED.md)), a Withings account, `withings-export auth login` completed.
+
+---
+
+## 1. Authenticate
+
+Withings uses OAuth2. Run the login flow once:
+
+```sh
+withings-export auth login
+```
+
+This opens a browser tab. Complete the OAuth consent and the token is stored at `~/.config/withings-export/auth.json`.
+
+Check readiness:
+
+```sh
+withings-export auth status
+```
+
+**Expected when logged in:**
+
+```
+logged in
+```
+
+Exit 0. **Expected when not logged in:**
+
+```
+Error: not logged in — run: withings-export auth login
+```
+
+Exit 1. `auth status` makes no network call — it only reads the local token file.
+
+**HTTPS callback workaround:** If your Withings OAuth app requires HTTPS, register `https://redirectmeto.com/http://localhost:8128/oauth/authorize` as the callback URL and set:
+
+```sh
+export WITHINGS_CALLBACK_URL="https://redirectmeto.com/http://localhost:8128/oauth/authorize"
+```
+
+---
+
+## 2. Orient with `prime`
+
+```sh
+withings-export prime
+```
+
+**Output** (reproduced at HEAD, 2026-05-19):
+
+```
+withings-export — primer for LLM agents
+=======================================
+
+WHAT IT IS
+  CLI for personal Withings data: activity, sleep, workouts, body
+  measurements, minute-level intraday samples (HR/HRV/SpO2/steps).
+
+I/O
+  stdout: data in --format markdown (default), json, or csv.
+  stderr: errors. Exit 0 on success including empty results.
+
+AUTH
+  withings-export auth login          OAuth2 in browser; tokens stored locally.
+  withings-export auth status         Exit 0 if usable, 1 with reason. No network call.
+  withings-export auth refresh|logout
+
+  Optional env: WITHINGS_CLIENT_ID, WITHINGS_CLIENT_SECRET, WITHINGS_CALLBACK_URL.
+  HTTPS-callback workaround: register https://redirectmeto.com/http://localhost:8128/oauth/authorize
+  (verbatim) and set WITHINGS_CALLBACK_URL to the same string.
+
+DATE FLAGS  (every subcommand)
+  --since VALUE / --until VALUE
+  VALUE: today | yesterday | YYYY-MM-DD | Nd/Nw/Nm/Ny
+  See https://github.com/quantcli/common/blob/main/CONTRACT.md#3-date-flags
+
+SUBCOMMANDS  (defaults in parens)
+  activity      (30d) daily steps/distance/calories/HR zones
+  sleep         (30d) stages, score, HR/RR; --derive polyfills missing nights
+  workouts      (90d) runs/walks/bikes/lifts with calories/HR/distance
+  measurements  (30d) weight/fat/BP/SpO2/temp; --types LIST filters
+  intraday      (1d)  minute-level HR/HRV/SpO2/steps; dense — keep windows narrow
+
+  Inspect any subcommand's row schema with: <subcommand> --since 1d --format json
+
+EXAMPLES
+  withings-export sleep --since 7d
+  withings-export workouts --since 30d --format json |
+    jq '.[] | {date, category, hr: .data.hr_average}'
+  withings-export measurements --since 30d --types 1 --format json |
+    jq 'sort_by(.date) | last'
+
+GOTCHAS
+  - Times are LOCAL; JSON epoch seconds are zone-agnostic.
+  - 'intraday' is a firehose — wide windows take minutes.
+  - Withings rate-limits aggressive callers (HTTP 601). 'sleep --derive' throttles itself.
+  - Sleep score / apnea fields appear only on supported devices.
+  - 'workouts.category' is an integer code in JSON; markdown/CSV map common codes to names.
+```
+
+---
+
+## 3. Export last week's activity
+
+```sh
+withings-export activity --since 7d
+```
+
+**Expected output** (markdown, one row per day):
+
+```
+| Date       | Steps | Distance (km) | Calories | Active (min) |
+|------------|-------|---------------|----------|--------------|
+| 2026-05-12 | 9240  | 6.8           | 2341     | 42           |
+| 2026-05-13 | 7110  | 5.2           | 2190     | 28           |
+```
+
+---
+
+## 4. Export sleep summaries
+
+```sh
+withings-export sleep --since 7d
+```
+
+For structured data including HR and sleep stages:
+
+```sh
+withings-export sleep --since 7d --format json | jq '.[0]'
+```
+
+---
+
+## 5. Get most recent body weight measurement
+
+```sh
+withings-export measurements --since 30d --types 1 --format json \
+  | jq 'sort_by(.date) | last'
+```
+
+`--types 1` filters to weight measurements (Withings measurement type 1). Omit `--types` to get all measurement types.
+
+---
+
+## 6. Export workout HR data
+
+```sh
+withings-export workouts --since 30d --format json \
+  | jq '.[] | {date, category, hr: .data.hr_average}'
+```
+
+`category` is an integer in JSON; markdown and CSV output maps common codes to names like `Running`, `Cycling`.
+
+---
+
+## 7. Check flag validation (contract §4, §7)
+
+These run without credentials (hermetic by [CONTRACT.md §7](https://github.com/quantcli/common/blob/main/CONTRACT.md#7-hermeticity)):
+
+```sh
+withings-export activity --help    # exits 0; no network call
+```
+
+**Expected:** help text including `--since`, `--until`, `--format` flags; exit 0.
+
+```sh
+withings-export activity --format lol 2>&1; echo "exit: $?"
+```
+
+**Expected:**
+
+```
+Error: invalid argument "lol" for "--format" flag: must be one of: markdown, json, csv
+exit: 1
+```
+
+Error on stderr, nothing on stdout, exit 1.
+
+---
+
+## 8. Run the contract conformance suite
+
+`withings-export` has the most complete compat coverage of the three CLIs: the suite stands up a stub HTTP server and a fake token so the data-path subtests run fully without real Withings credentials.
+
+```sh
+git clone https://github.com/quantcli/withings-export-cli
+cd withings-export-cli
+go build -o /tmp/withings-export .
+WITHINGS_EXPORT_BIN=/tmp/withings-export go test -tags=compat ./...
+```
+
+**Expected:**
+
+```
+ok      github.com/quantcli/withings-export-cli    0.003s
+ok      github.com/quantcli/withings-export-cli/internal/auth    0.003s
+```
+
+The suite covers CONTRACT.md §4 (format flag surface, `--format json` returns a JSON array, `--format csv` returns a header row, default equals `--format markdown`) and §7 (hermeticity) across all five data subcommands. All cells in the CONTRACT.md Status table for `withings-export` are **machine**-attested.
+
+---
+
+## What to look at next
+
+- `withings-export intraday --help` — minute-level HR/HRV/SpO2 (keep windows narrow)
+- `withings-export prime` — jq recipes and rate-limit gotchas
+- [CONTRACT.md §3](https://github.com/quantcli/common/blob/main/CONTRACT.md#3-date-flags) — date flag semantics
+- [CONTRACT.md §4](https://github.com/quantcli/common/blob/main/CONTRACT.md#4-output-format) — output format contract (`csv` is withings-only today)
+- [crono-export example](https://github.com/quantcli/crono-export-cli/blob/main/docs/example.md) — if you also track nutrition
+- [liftoff-export example](https://github.com/quantcli/liftoff-export-cli/blob/main/docs/example.md) — if you also track gym workouts