fix: Codex description limit + wrong-repo bug

Move skill routing table from root SKILL.md.tmpl description (1017/1024
chars) to body where there's no length limit. Add 900-char warning
threshold test. Add -C flag to all codex exec calls so Codex always
runs in the correct git root directory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SKILL.md.tmpl

@@ -20,28 +20,6 @@ If `PROACTIVE` is `false`: do NOT proactively suggest other gstack skills during
 Only run skills the user explicitly invokes. This preference persists across sessions via
 `gstack-config`.
 
-If `PROACTIVE` is `true` (default): suggest adjacent gstack skills when relevant to the
-user's workflow stage:
-- Brainstorming → /office-hours
-- Strategy → /plan-ceo-review
-- Architecture → /plan-eng-review
-- Design → /plan-design-review or /design-consultation
-- Auto-review → /autoplan
-- Debugging → /investigate
-- QA → /qa
-- Code review → /review
-- Visual audit → /design-review
-- Shipping → /ship
-- Docs → /document-release
-- Retro → /retro
-- Second opinion → /codex
-- Prod safety → /careful or /guard
-- Scoped edits → /freeze or /unfreeze
-- Upgrades → /gstack-upgrade
-
-If the user opts out of suggestions, run `gstack-config set proactive false`.
-If they opt back in, run `gstack-config set proactive true`.
-
 # gstack browse: QA Testing & Dogfooding
 
 Persistent headless Chromium. First call auto-starts (~3s), then ~100-200ms per command.