chore: update stale documentation

[Lucas-Bur]

Problem Statement

Several documentation files are stale and contain outdated information:

• docs/OPEN-QUESTIONS.md — most questions have been answered but file was never updated
• docs/rules/behavior.md — checkboxes marked [ ] that appear to already be done
• docs/adr/0003-hexagonal-architecture.md — says "ResetIndex use case not yet implemented" but it exists
• README.md — mentions "ffmpeg scanning" in architecture section but ffmpeg is not used

What to build

Update all stale documentation to reflect current state of the codebase.

Acceptance criteria

• OPEN-QUESTIONS.md updated — answered questions moved to "Resolved" section or removed
• behavior.md checkboxes updated to match reality
• adr/0003 updated — ResetIndex note removed
• README.md — "ffmpeg scanning" reference corrected
• No other stale references found in docs

Blocked by

None - can start immediately

[Lucas-Bur]

Cross-link: docs touchpoints for Effect v4 / TS-Go

Tracking the docs work that overlaps with #139 (chore: harden Effect v4 stack + switch to TypeScript-Go).

Files to update when #139 lands:

• CONTEXT.md — add a "Test runner" glossary entry (decision: vite-plus/test vs @effect/vitest); note that v4 LS lives in @effect/tsgo
• docs/rules/behavior.md — drop v3 @effect/language-service references; add @effect/tsgo LSP note
• docs/OPEN-QUESTIONS.md — close out "which TypeScript?" and "which Effect version?" questions (resolved by chore: harden Effect v4 stack + switch to TypeScript-Go before release #139)
• docs/adr/0001-effect-runtime.md — confirm still matches v4 ("smol") or update
• docs/adr/0004-ci-cd-pipeline.md — update the lint:effect invocation example to effect-tsgo diagnostics
• README.md — verify badges / build status reflect new toolchain

Verification: vp check --fix already ran clean on 2026-06-30 (133 files, 0 warnings) — formatting was the only blocker, now resolved.