docs(dispatcher): remove stale legacy Identity absence note - #16380

Summary

Remove the stale legacy-absence claim from Session.identity field 16. The protobuf field number and Identity fencing are unchanged.

Per-line audit

• dispatcher/README.md (around line 99): kept. Migration 18 does not delete a Session only because its ServiceAccount owner columns are NULL; it deletes rows without execution_identity_name or without caller attribution. The current Identity-run Session path can retain an exact execution Identity and attribution without a managed ServiceAccount. A same-named ServiceAccount does not inherit access to that Session.
• dispatcher/SPEC.md (around line 133): kept for the same live shape. Current Session creation records a complete execution Identity, while the ServiceAccount owner columns can remain NULL. Migration 18 retains rows with execution Identity and attribution even when those columns are NULL.
• session_dispatcher.proto field 16: rewritten. Migration 18 deletes rows with a missing execution Identity. Current CreateSession requires a complete Identity, Store creation rejects an incomplete Identity, and scanSession rejects missing or incomplete stored Identity before the response projection. No current response can omit field 16 because of a pre-cutover row. The new comment makes no Sandbox, Sandbox UID, or single-environment requirement.

I checked agentplane/plans/session_environment_model.md and the REFACTOR_COHERENCE_AUDIT entry in agentplane/plans/task_dag.md. The selected central Session design allows zero or multiple environment attachments, and this change does not make an environment mandatory.

Deliberate exclusions

The README and SPEC owner-column lines stay because that nullable ServiceAccount-owner shape is still possible for Identity-run Sessions. Notification Service owner_namespace/owner_name columns are unrelated and untouched.

Tests

No system test specifically covers these comments. Existing Dispatcher integration tests cover Identity-run Sessions and the Identity response projection. This is a comment-only change; no tests were run. pre-commit run --files agentplane/runner_gateway/dispatcher/api/session/session_dispatcher.proto passed.

Questions for Claude

GitHub reports auto-merge enabled on #16380 by the repository account. I tried to disable it with the authorized agentydragon-agent token, but GitHub returned FORBIDDEN for disablePullRequestAutoMerge. Please disable auto-merge manually so this PR remains for hand review and merge.