Files Changed: session-bind Sidebar external file mutations

Acceptance Criteria

• With an active session and valid reservation, an eligible Sidebar file operation prepares before mutation and commits only after filesystem success.
• A failed post-prepare filesystem operation aborts and never creates an owned changed row.
• Cancelled input and delete confirmation perform neither prepare nor mutation.
• With no active chat session, draft route, unavailable server, or rejected reservation, ordinary Sidebar behavior remains intact but every session leaves the operation untracked.
• The operation uses the last focused chat panel's session, never a primary panel or project-path guess.
• A rename or move submits distinct source and destination targets; the server owns their canonical identities.
• Watcher invalidation carries no client-derived modified or deleted status and triggers assessed-detail refetch.
• Successful filesystem work remains visibly successful if later commit transport fails; tracking is aborted best-effort rather than falsely reported.
• Existing trash behavior, collision checks, native editor opening, and legacy in-worktree diff behavior remain unchanged.

Testing Decisions

• Extend Sidebar bridge and file-operation tests with prepare-before-mutate, commit-after-success, abort-after-failure, no-session fallback, and two-target move cases.
• Extend chat bridge and multi-panel tests for per-panel session context, draft and close clearing, and last-focused-panel selection.
• Extend relay coverage for normal and transition iframe session-context and reassessment envelopes.
• Add focused watcher coverage for canonical descriptor registration, opaque-reference invalidation, debounce, and stale-watch disposal.
• Extend assessed-detail bridge coverage so invalidation refetches server evidence instead of applying local status overrides.

Key Decisions

• Reservation capabilities stay extension-local and are never persisted or sent through the Sidebar webview.
• Watchers are invalidation only; the server determines changed, unchanged, or unavailable.
• Directory operations stay behavior-preserving and untracked until recursive capture has its own contract.
• Client rename maps can remain for legacy behavior but are never authoritative for registered external references.

Constraints & Invariants

• The extension never determines externality with lexical path checks.
• No baseline bytes, patches, expected digests, or raw canonical paths enter browser tracking state.
• No active session or invalid capability is untracked, never silently attributed.
• The OpenCode fork remains source of truth and the overlay is synchronized from it.

Prior Art

• Parent Files Changed: full session-lineage mutation ledger #972
• fix(app): trust server session diff for Files Changed panel #720, Slice 1: Contamination fix — derive agent-file filter from tool filediff only #742, Slice 2: External-change reactivity — watcher ungating + client event handler #743, Files Changed: per-session file watcher for reliable external-change refresh #844, and Files Changed: per-session file watcher for reliable external-change refresh #845
• Existing Sidebar operations, chat bridge, and watcher bridge

Source

Part of #972. Blocked by #975.

Notes

This child starts as hitl because the parent review is degraded. Its own deliberate review must pass before implementation begins.