Phase 7.3: completion pill + per-session JSON audit log

What was done

Two pieces that were deferred from Phase 7.1 landed together as Phase 7.3:

1. presentCompletion pill — a green (RGB 0.2, 0.65, 0.3) checkmark.circle.fill pill on the HUD that auto-dismisses after 2.5s when a task completes. Replaces the prior reuse of presentError for the "we're done" state.
2. SessionAuditLog — per-session JSON dump written to ~/Library/Application Support/com.screenbridge.app/sessions/<uuid>.json. Captures the instruction, every step's target/action/source/timing, and the final outcome. This is Layer 5 of the 5-layer security architecture — the user-visible "what did ScreenBridge actually see/do?" record.

Why it was needed

From the commit message: Phase 7.1 deferred both items; this commit picks them up for dogfooding UX. The completion pill exists because reusing the error pill for "task complete" was wrong-signal (red box on success). The audit log exists for three stated purposes: debug retro, user trust ("Open sessions folder" menu-bar item already wired), and pattern extraction for the mother-dogfooding scenario — figuring out where a session gets stuck.

Files / functions changed

• Sources/ScreenBridge/HUDOverlayView.swift — new HUDContent.completion case + CompletionPill view (headline font, 462pt max width).
• Sources/ScreenBridge/HUDController.swift — presentCompletion(message:on:autoDismissAfter:), plus a currentContent property guarding against the auto-dismiss racing a new task's content.
• Sources/ScreenBridge/AppDelegate.swift — processAnalyzeStage swaps presentError for presentCompletion on task complete, with the Korean-friendly default message "끝났어요!" when nextAction is empty.
• Sources/ScreenBridge/SessionAuditLog.swift (NEW, 88 lines) — SessionAuditEntry, StepRecord, Outcome enum (inProgress / completed / cancelledByUser / cancelledByTimeout / error), save() (atomic write, prettyPrinted + sortedKeys + ISO8601), load() for debug.
• Sources/ScreenBridge/AnalyzeCoordinator.swift — wires the audit entry: new entry on fresh task, append + save on every step success, finalize with outcome on taskComplete or cancelSession.
• DECISIONS.md — records R9 Phase 7.3 decisions: per-session JSON (vs append-only log / SQLite) and green checkmark pill (vs ErrorPill reuse).

180 LOC across 6 files.

Result / verification

From the commit message: swift build 2.69s, swift test 116/116 passing (unchanged — audit save is a best-effort side effect that unit tests don't reach). The dogfooding handle described in the commit: run "chrome 켜고 naver 가기" to completion and a green "끝났어요!" pill appears for 2.5s; mid-session, the <uuid>.json file is already written under ~/Library/Application Support/com.screenbridge.app/sessions/ and inspectable via the existing menu-bar "Open sessions folder".

Safety invariants held: no screenshot in the JSON (bounded growth), audit save is best-effort and silent on failure so it can't block a session, and the file path stays local — never cloud-synced (called out as a Phase 7.0 synthesis invariant in the file header comment).

Open / what didn't get done

• 7-day automatic deletion of session JSON files is marked TODO Phase 7.4 in the file header.
• Audit save is best-effort by design — there is no surfaced UI when a save fails (only an Log.app.notice).
• No unit tests for SessionAuditLog itself; coverage relies on dogfooding the menu-bar "Open sessions folder" path.

Commit — a54b121