fix: clear stale entity metadata during ACP v2 replay - #716

Summary

• Extend v2 replay initialization from messages to tool calls, terminals, and compactions. The first upsert for each entity explicitly clears retained _meta once per session replay. If the report has current metadata, clear first and then restore that object in a separate update.
• Apply initialization after v2 conversion, where a command report becomes both terminal and tool updates. Their IDs may match, but they have separate entity namespaces and each needs its own clear.
• Generalize the existing message-primer send path rather than adding separate reset logic to every reporter. Standard, AIR/native, root, and child history paths share it, including paginated history and repeated replay on the same connection.
• Keep shared publication fencing and tool-report filtering/tracking before conversion. Completed-tool tracking still suppresses duplicate completions and late output, while subsequent partial and live updates retain normal metadata-patch semantics.
• Leave chunk metadata, plan metadata, session-level metadata, and v1 wire behavior unchanged. Only three existing v2 snapshots gain the intended metadata clears.

Checked against ACP 8eb7e749, especially docs/protocol/v2/extensibility.mdx:103-150 and docs/protocol/v2/session-setup.mdx:312-317. Replay does not implicitly reset metadata. Empty current metadata requires _meta: null; a nonempty replacement needs an explicit clear before the current object when stale keys are unknown.

Scope

This is replayed-entity metadata synchronization, not reconstruction of session-wide extension metadata or other omitted entity fields. Session-level metadata remains separate because its writers also publish live extension state. The newer upstream auth/status schema and draft additions encountered during the specification refresh are not adopted in this PR. No review/checklist Markdown file is included.

Validation

• npm run typecheck
• npm run build
• npm test: 2,346 passed, 39 skipped
• npm run test:integration: all 19 passed
• npm run codex-test -- -p "Reply with exactly OK. Do not use tools or modify any files." -o summary: completed with end_turn
• git diff --check
• Opt-in E2E tests were not run.

Tests cover retained metadata removal, restoring current metadata including nested null values, keeping fresh metadata through later status/partial updates, separate tool/terminal identities, preserved message primers, untouched chunk/plan/session metadata, completed-tool filtering and late-output suppression, and post-close publication fencing. Wire tests exercise two replays on one connection across standard/AIR and legacy/paginated history. Existing v1 and child-ordering snapshots remain unchanged.