fix(hooks): detect a pc installed from the retired Homebrew formula

[mcdgavin]

Problem

hooks/pinecone-api-key-check.sh decides whether the Pinecone CLI is installed with command -v pc. The Homebrew formula pinecone-io/tap/pinecone was disabled on 2026-03-30 when the CLI moved to a cask, but a formula installed before then keeps a working pc. Those users read as "installed" every session, so neither they nor Claude hear that the install is stranded, and their next brew upgrade fails on the disabled formula with no warning from the plugin.

This is the gap noted as a follow-up in #46, which fixes the install string itself. The two PRs are independent and touch different hunks of the hook.

Fix

Homebrew links a formula binary into Cellar/ and a cask binary into Caskroom/, so one readlink on the resolved pc path tells them apart:

case "$(readlink "$(command -v pc)" 2>/dev/null)" in
  *Cellar/pinecone/*) cli_via_formula="yes" ;;
  *) cli_via_formula="no" ;;
esac

When a formula install is detected:

• Agent context gains a note saying the install still runs but is stranded, telling Claude not to interrupt the user for it, and giving the migration command to suggest the next time the CLI comes up: brew uninstall pinecone-io/tap/pinecone && brew install --cask pinecone-io/tap/pinecone.
• User message gains one clause: "Heads-up: pc came from the retired Homebrew formula; ask Claude how to move it to the cask."

A manual install is not a symlink, readlink prints nothing, and it is treated as a normal install. No new dependencies; readlink is POSIX and Homebrew already assumes it.

Verification

The disabled formula cannot be installed any more, so the formula case is simulated with a temp prefix laid out the way Homebrew does it: prefix/bin/pc -> ../Cellar/pinecone/0.3.1/bin/pc, with the fixture first on PATH.

PATH state	installed =	Note emitted	JSON valid
Simulated formula-linked pc	yes	yes, in both context and user message	yes
Real cask-linked pc on macOS, Homebrew 6.0.22 (/opt/homebrew/Caskroom/pinecone/1.0.1/pc)	yes	no	yes
No pc on PATH	no	no, existing install guidance unchanged	yes

bash -n passes. Output for the cask and absent cases is byte-identical to before.