fix: live cumulative diff during multi-step turns

[jeonghun-jj-lee]

Follow-up to #181 — fixes the Files Changed panel going stale during live multi-step workflows.

Problem

After #181, the cumulative diff worked at turn boundaries but broke mid-turn:

1. The query only refetched on session status transitions (idle↔busy), not between individual edits
2. Session.diff() used a stale lastStepFinish snapshot as the diff endpoint instead of tracking the live working tree
3. Files outside the workspace root (e.g., editing opencode source from an Altissimo workspace) produced no diff at all — the snapshot system only tracks the workspace worktree

Fixes

A. Prefer live working tree (session.ts)

// Before (stale during live turns):
const to = lastStepFinish ?? (yield* snapshot.track())
// After:
const to = (yield* snapshot.track()) ?? lastStepFinish

B. Refetch diff query on each completed edit (packages/app)

Added a diff_version counter per session that bumps when a file-editing tool completes (via SSE message.part.updated). The query key includes this counter → refetch after every edit (~100-250ms, acceptable at edit frequency).

C. External file support (snapshot/index.ts + session.ts)

For files outside the tracked worktree:

• Resolve ../ relative paths to absolute
• Find the file's own git repo (git rev-parse --show-toplevel)
• Look up what commit was HEAD at session start (git rev-list -1 --before=<time>)
• Diff that committed state against current disk
• Display with ~/ prefix (not the broken ../../../ path)

Falls back to tool-metadata accumulation for files not in any git repo.

Files changed

Package	File	Change
opencode	src/session/session.ts	Fix A + C: track() priority, external file splitting
opencode	src/snapshot/index.ts	Fix C: diffExternalFiles(files, startTime)
app	src/pages/session.tsx	Fix B: diff_version in query key
app	src/context/global-sync/event-reducer.ts	Fix B: bump on completed edit parts
app	src/context/global-sync/types.ts	Fix B: diff_version field
app	src/context/server-session.ts	Fix B: store init
app	src/context/global-sync/child-store.ts	Fix B: store init
app	src/context/directory-sync.ts	Fix B: session field list
app	src/context/global-sync/session-cache.ts	Fix B: cache cleanup
app	tests	Fix B: fixture updates