fix(hooks): make pstack opt-in on Claude Code

[thisguymartin]

Closes #18

What changed

• plugins/pstack/hooks/session-start-context.md replaces the auto-fire mandate with an opt-in gate. Claude invokes a pstack:* skill only when the user types /pstack:<name>, names pstack or one of its skills, or keeps a standing CLAUDE.md or AGENTS.md instruction for it. A pstack skill the user started keeps routing to the skills it needs, and dispatched subagents follow their dispatch prompt.
• The gate also covers skills whose descriptions ask for automatic use (unslop, typescript-best-practices). It does not add disable-model-invocation, which would block poteto-mode from invoking them.
• Docs: README.md, docs/reference.md, docs/USAGE.md, NOTICE.md, CHANGES.md, and UPSTREAM-FLEX.md describe the opt-in behavior, the CLAUDE.md line that restores always-on routing, and the new fork-owned conflict surface.
• Codex has no plugin hook runtime, so only its docs change.

Verification

• Bun tests, strict typecheck, static invariants, and plugin validation pass.
• The exact candidate is installed in every affected harness.
• The changed behavior passes from each real user surface.
• The installed version, action, and observed result appear below.

Local gate: bun run test 198 pass, 0 fail. bun run typecheck clean. Manifest JSON parse ok. PSTACK_STATIC_ONLY=1 bash tests/skill-collision-repro.sh all ok. claude plugin validate plugins/pstack passed.

Pre-install headless probes (claude -p, Claude Code 2.1.287, scratch repo with a total() that ignores qty):

Plugin	Prompt	First pstack Skill call
Installed 1.4.1 (old mandate)	Fix the qty bug, add a test	pstack:tdd
This branch via --plugin-dir	Same	none; edited cart.ts directly
This branch via --plugin-dir	Add discount codes (2 runs)	none in either run
This branch via --plugin-dir	"Use pstack for this." + qty bug	pstack:poteto-mode, then read playbooks/bug-fix.md

Live evidence:

Pending.

A pull request without live evidence remains a draft. Do not merge, tag, release, or roll it out.