refactor: index-hook framework — eliminate per-pass CodeGraph mutations

[mschreib28]

Reviewers: for a 5-minute review path that surfaces only what to verify, see the review checklist comment below. The original description is preserved here for full technical detail.\n\n---\n\nrefactor: index-hook framework — eliminate per-pass CodeGraph mutations\n\nToday every PR adding a derived-signal pass (centrality, churn,\nissue-history, config-refs, sql-refs, cochange) edits the same\n3 spots in src/index.ts:\n\n 1. New imports at the top\n 2. New private method on CodeGraph (e.g. runDerivedSignals,\n runIssueHistoryPass, runConfigRefsPass, runSqlRefsPass)\n 3. New call site in indexAll AFTER resolution\n 4. New call site in sync AFTER resolution\n\n5 PRs collide on every one of those.\n\nAfter this refactor:\n\n Adding a new derived-signal pass:\n 1. Create src/index-hooks/<name>.ts exporting a\n HOOK: IndexHook constant with afterIndexAll and/or\n afterSync methods.\n 2. Add one import + one entry to\n src/index-hooks/registry.ts.\n\nCodeGraph.indexAll and sync invoke the hook runner once;\nadding a new pass touches only the hook file + the registry.\nZero changes to CodeGraph itself.\n\n## What's new\n\n- src/index-hooks/types.ts — IndexHook interface\n (afterIndexAll, afterSync, both optional), IndexHookContext\n (projectRoot + config + queries + db), and\n IndexHookOutcome for diagnostic reporting.\n- src/index-hooks/registry.ts — static-import list of every\n registered hook (empty on main today; PRs adding hooks fill it\n in), plus the runAfterIndexAll / runAfterSync runners that\n iterate hooks and catch errors so one broken hook never fails\n indexing.\n- src/index.ts — indexAll calls runAfterIndexAll(ctx)\n after resolution. sync calls runAfterSync(ctx, result)\n after resolution. New private buildHookContext() helper\n exposes a stable read-only context.\n- tests/index-hooks.test.ts — 6 tests covering empty\n registry, runner shape, and the afterIndexAll / afterSync\n contracts.\n\n## Why ship the framework on main with zero registered hooks?\n\nThe only consumers of this framework today are 5 unmerged PRs\n(colbymchenry#105 cochange + my colbymchenry#112-colbymchenry#115). Landing the framework now lets\neach of those PRs rebase to a 2-line change instead of 8-10\nlines mutating CodeGraph adjacent-line. Without this, all 5 PRs\ncollide on the same indexAll/sync call sites.\n\nThe framework adds zero behavior on main (no registered hooks =\nno-op runner). 380→386 tests confirm no regression.\n\n## Affected open PRs\n\n| PR | Today | After this lands |\n|---|---|---|\n| colbymchenry#105 cochange | runDerivedSignals helper + 2 call sites | 1 hook file in src/index-hooks/ + 2 lines in registry.ts |\n| colbymchenry#112 centrality+churn | same shape | same shape |\n| colbymchenry#113 issue-history | same shape | same shape |\n| colbymchenry#114 config-refs | same shape | same shape |\n| colbymchenry#115 sql-refs | same shape | same shape |\n\nEach goes from "edit CodeGraph in 4 spots" to "drop a hook file."\n\nCo-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com\n

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Copied from colbymchenry/codegraph#119