v0.5.4

chore: merge AGENTS.md with retch, add just pr gate (#47)

* chore: merge AGENTS.md with retch, add just pr gate

Restructure AGENTS.md into a Portable Core (kept in sync with retch's
AGENTS.md) plus project-specific rules, adopting retch's WIP.md
handoff workflow and branch-cleanup rule. Add just pr/merge-pr
recipes to automate the Pre-PR checklist, mirroring retch's gate.

Assisted-By: Claude Sonnet 5

* fix: clarify just pr man-page step wording in AGENTS.md

The step 4 description contradicted itself, claiming man/build/
needed to be regenerated and committed while also correctly noting
it's gitignored. Clarify that this step only proves the build
succeeds since there's nothing to diff.

Assisted-By: Claude Sonnet 5

* fix: repo-relative CLAUDE.md link, require reading ~/AGENTS.md

CLAUDE.md hardcoded an absolute path to AGENTS.md that only resolved
on a clone at exactly ~/git/etr; switch to a relative link. Also add
a Global Mandates item to the Portable Core requiring agents to read
~/AGENTS.md before starting work, so cross-repo mandates aren't
silently skipped when only the repo AGENTS.md is consulted.

Assisted-By: Claude Sonnet 5