feat: add mix setup/git_hooks, activate a pre-push conventional-commit hook

[bougyman]

Summary

• mix git_hooks sets core.hooksPath to githooks/ (idempotent). mix setup runs it, and is expected to grow more steps over time as more repo-management tasks land in this project.
• Adds a new pre-push hook alongside the existing commit-msg one - reuses ci/conventional_commits.sh as-is (no changes needed: with no env vars set it already falls back to main/origin/main as base and the current branch as head, exactly right for a local push) to catch anything that slipped past commit-msg (a commit made before the hooks were installed, an amend, etc.), not just the current HEAD.
• Removes app/lib/mix/tasks/githooks.install.ex - an exact duplicate of this same idea living in app/'s own mix project instead of the new top-level one.
• Updates Readme.adoc/AGENTS.md/app/usage-rules.md's references to the old manual git config core.hooksPath githooks / mix githooks.install steps to point at mix setup instead.

Test plan

• mix setup run twice - installs cleanly, idempotent (identical output, no error)
• githooks/pre-push run directly - exits 0 against this branch's own commits
• The underlying validation logic is already proven to catch bad subjects for real: this exact ci/conventional_commits.sh failed CI on PR ci: catch and auto-fix AGENTS.md usage-rules drift #88's badly-worded merge commit earlier today
• mix test/mix format --check-formatted clean in both projects (root and app/)
• mix usage_rules.sync --check still passes after editing app/usage-rules.md
• Confirmed no remaining references anywhere to the removed githooks.install task

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds a root-level mix setup workflow to install repo git hooks and expands Conventional Commit enforcement with a new pre-push hook, while updating documentation to point contributors at the new setup flow.

Changes:

• Add mix setup and mix git_hooks tasks to configure core.hooksPath to githooks/ idempotently.
• Add a githooks/pre-push hook that runs ci/conventional_commits.sh to validate the whole push range.
• Remove the duplicate app-scoped mix githooks.install task and update docs to reference mix setup.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Readme.adoc	Updates contributor instructions to use mix setup for hook activation.
lib/mix/tasks/setup.ex	Introduces a single entrypoint task intended to grow with additional repo setup steps.
lib/mix/tasks/git_hooks.ex	Adds an idempotent Mix task to set core.hooksPath to githooks.
githooks/pre-push	Adds a pre-push Conventional Commit validation hook using the existing CI script.
app/usage-rules.md	Updates usage rules documentation to reflect both commit-msg and pre-push hooks and mix setup.
app/lib/mix/tasks/githooks.install.ex	Removes the duplicate/legacy hook-install Mix task from the app/ project.
AGENTS.md	Updates agent guidelines to reference the new hook activation flow.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.