fix(opencode): emit OpenCode 2.x plugin definition with id and setup

[ranokay]

Closes #1038.

Problem

The OpenCode installer template (_opencode_plugin_content() in code_review_graph/skills.py) exports a bare function using the removed app.on API. On OpenCode 2.x (verified v2.0.7) the loader rejects it before any hook runs:

PluginModule.LoadError: Plugin must export a default definition with an id and an effect or setup function. (cause: SchemaError(Expected object at ["default"]))

opencode plugin list shows the entry with ID - / state failed. #947 covers the app.on failure; #949 ports the template to the V1 returned-hooks shape, but a function default export still fails the V2 loader, so 2.x stays broken.

The template's run($, cmd) helper has a second bug (also noted in #947): interpolating a full command string into Bun's $ template escapes it into a single executable name.

Fix

• Template now emits a V2 definition: Plugin.define({ id: "code-review-graph", setup(ctx) {...} }) from @opencode/plugin.
• session.created via ctx.event.subscribe + status (scoped to the plugin instance directory).
• File edits via ctx.tool.hook("execute.after") on edit/write/patch + update --skip-flows (V2 has no file.edited hook).
• Git-commit detection via ctx.tool.hook("execute.before") on bash/shell/execute + detect-changes --brief, never blocking.
• Subprocesses via node:child_process execFile with argv arrays.
• Tests updated to the V2 shape; CHANGELOG entry added.

Verification

• uv run pytest tests/test_skills.py -q: 210 passed.
• uv run ruff check code_review_graph/skills.py tests/test_skills.py: clean.
• Generated template bun build --external @opencode/plugin: bundles clean.
• Live run on OpenCode v2.0.7: after opencode reload, opencode plugin list and /api/plugin report code-review-graph -> active (was -/failed), no new failed to load plugin entries; code-review-graph status works against a real graph.

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[tirth8205]

This no longer merges into staging. The one conflict is CHANGELOG.md, and the branch is 51 commits behind.

All of it is prose, so taking staging's text and re-applying your lines on top should be the whole job.

git fetch origin
git merge origin/staging
# resolve, then
uv run pytest tests/ -q
uv run ruff check code_review_graph/
uv run mypy code_review_graph/ --ignore-missing-imports --no-strict-optional
git push

I have not reviewed the change itself yet. That comes once it merges and the checks run against the merged state, since staging has moved a long way and the result is what matters.