feat(pi-extension): autocomplete /ponytail arguments

[ruttybob]

Summary

 pi.registerCommand("ponytail", {
   description: PONYTAIL_COMMAND_DESCRIPTION,
+  getArgumentCompletions: (prefix) => completePonytailArgs(prefix),
   handler: async (args, ctx) => ...
 })

completePonytailArgs(prefix)
  split prefix into words
  1 word  → filter completion table by prefix
            (lite, full, ultra, off, status, default)
  "default <…>" → complete runtime levels on the 2nd word,
                  insert "default <mode>"
  extra words / no match → null

Mirrors the built-in /mcp command completion pattern (startsWith filter,
null when nothing matches, trailing space after default so the second
level opens immediately). review is deliberately not offered: it is
session-only and rejected by parsePonytailCommand (#377).

Evidence

• Before: typing /ponytail d offers nothing; default accepts modes users cannot discover.
  After: /ponytail d → default → /ponytail default → off/lite/full/ultra.
• npm test in pi-extension/: 29 pass, 0 fail, including the new
  /ponytail autocompletes modes, commands and default <mode> (8 assertions:
  full list, prefix filter, both default levels, no-match → null).
• node scripts/check-rule-copies.js ✅ · node scripts/check-versions.js ✅
  Fixes pi-extension: argument completion for /ponytail #1081
• Note: npm test at the repo root has one pre-existing failure on main
  (csv: correct pandas one-liner passes) — the local env lacks pandas,
  documented in CONTRIBUTING. Verified it fails on a clean tree via git stash.

Merge Danger

Door: two-way — additive, UI-only; Pi versions without
getArgumentCompletions ignore the field.

Blast Radius: pi-extension only.