Address remaining PR #158 review comments (all fixed in de6db84)

[Copilot]

PR #158 was merged before all reviewer comments were actioned. All five outstanding items were addressed in PR #159 (commit de6db84), which is already included in this branch.

Changes (de6db84)

• brief-intent.ts – deriveSeedIds() now normalizes relative editedFilePath to absolute via path.isAbsolute() / path.join(workspaceRoot, ...) before calling getSourceLinkedRequirementIds(), fixing silent [] returns when file.edited emits a relative path
• index.ts – computeBriefIntent() called once in file.edited handler; fingerprint cached in lastBriefFingerprint. system.transform hook reads from cache instead of recomputing — eliminates redundant symbols.yaml I/O on every transform
• Docs – Removed accidental #YT|> annotation prefixes from REQ-opencode-kibi-briefing-v1.md, SCEN-opencode-kibi-briefing-v1.md, and TEST-opencode-kibi-briefing-v1.md

// Before: relative path passed directly, silently returns []
return getSourceLinkedRequirementIds(params.workspaceRoot, params.editedFilePath)

// After: normalized to absolute before lookup
const absoluteEditedPath = path.isAbsolute(params.editedFilePath)
  ? params.editedFilePath
  : path.join(params.workspaceRoot, params.editedFilePath);
return getSourceLinkedRequirementIds(params.workspaceRoot, absoluteEditedPath)