feat(kb): Comment for Scout — marker vocabulary, parser chip, block affordance (KB phase)

[jordanrburger]

Implements the approved "Comment for Scout" (KB phase) design + plan from #72 (docs/superpowers/specs/2026-07-06-kb-comment-for-scout-design.md, docs/superpowers/plans/2026-07-06-kb-comment-for-scout.md), following the plan's 3-task TDD sequence.

What

Select (hover) a block in a Knowledge Base note in Read mode and leave a comment for Scout to act on during its next dreaming session. The comment is persisted as an inline //==<< … >>==// marker line beneath the block via the existing guarded + git-committed KnowledgeBaseFileWriter.save path. Dreaming already consumes and strips these markers — no plugin-side changes.

Tasks (as planned)

1. ScoutMarker vocabulary (Scout/KnowledgeBase/Models/ScoutMarker.swift) — single source of truth for the marker syntax: format (trims, collapses newlines, rejects empty), isMarkerLine, body(of:). 6 unit tests.
2. Parser support (KBDocSegment) — new .scoutComment kind; a standalone marker line parses as its own segment (never absorbed into an adjacent paragraph or table row; a marker inside a code fence stays code). New whole-line splicers insertLine(in:afterLineEnd:line:) / removeLines(in:start:end:) — only ever add/remove whole lines, so every other byte (incl. plugin structured tokens) stays untouched. 9 unit tests.
3. Editor affordance (KBEditableView / KBEditorView) — hover a block → "💬 Comment for Scout" → inline composer (Cancel / Send to Scout, ⌘↵, empty-input disabled). Send splices the marker after the block and persists immediately through KBEditorView.save() (baseline conflict guard + scoped git commit + .commitFailed handling all reused as-is). Marker segments render as a "for Scout · pending" chip with a × to retract (whole-line delete + save). When dreaming strips the marker, the FSEvent reparse makes the chip disappear.

Scope guards (per spec)

• KB phase only — the Action Items counterpart is deferred to scoutctl: add action-items marker command (//==<< … >>==//) for app-authored Scout feedback scout-plugin#186 (needs a scoutctl marker command).
• No plugin-side changes; no new writer op; block-granularity only; markers are transient by contract (no sidecar store).

Verification

• Full ScoutTests target: 438 tests in 79 suites, 0 failures (platform=macOS).
• Debug build succeeds; Debug app launch smoke-tested.
• Not yet done — manual UI walkthrough (plan Task 3 Step 8): hover → composer → send → chip → rg -F '//==<<' shows the marker on its own line → scoped commit diff adds exactly one line → × retract removes it. This needs a human driving the app; @jordanrburger please run this before merging.

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