feat(session-context): resolve the active path per AI_AGENT_ID

A single .ai/session-context.org races when two agents share a project: each agent's writes clobber the other's session log. I added .ai/scripts/session-context-path, which resolves the active path from AI_AGENT_ID: unset gives the legacy .ai/session-context.org singleton (so every existing one-agent session is unchanged), set gives .ai/session-context.d/<id>.org with the id sanitized to filename-safe characters. This is Codex's Phase 1 slice from the runtime-neutral spec: the race fix on its own, no broader refactor.

startup.org's existence check and wrap-it-up.org's rename now resolve through the helper, each with a singleton fallback so older checkouts that haven't synced the script still work. Wrap folds the agent id into the archive name so two agents wrapping in the same minute don't collide. protocols.org documents the rule. Verified with 5 bats cases and a two-agent simulation showing distinct paths per id.

1 files changed, 3 insertions, 1 deletions

diff --git a/todo.org b/todo.org
index 5b7c924..dbe3820 100644
--- a/todo.org
+++ b/todo.org
@@ -1197,11 +1197,13 @@ The original handoff also noted a related anomaly: even with =--delete=, two fil
 
 Source: =inbox/2026-05-29-0832-from-jr-estate-investigate-startup-rsync-carried-dirty.org= (processed and deleted).
 
-** TODO [#B] Codex Phase 1 — AI_AGENT_ID + session-context.d/<id>.org :feature:
+** DONE [#B] Codex Phase 1 — AI_AGENT_ID + session-context.d/<id>.org :feature:
+CLOSED: [2026-05-30 Sat]
 :PROPERTIES:
 :CREATED: [2026-05-28 Thu]
 :LAST_REVIEWED: 2026-05-28
 :END:
+Shipped backward-compatibly. New =.ai/scripts/session-context-path= helper resolves the active path from =AI_AGENT_ID=: unset → the legacy =.ai/session-context.org= singleton (one-agent default unchanged, per the spec's compatibility rule), set → =.ai/session-context.d/<sanitized-id>.org=. startup.org's existence check and wrap-it-up.org's rename now resolve through the helper (with a singleton fallback for older checkouts); wrap folds the agent id into the archive name. protocols.org documents the rule. Verified: 5 bats cases + a two-agent simulation showing distinct paths per id. Larger runtime-neutral arc (runtimes/ manifests, launcher refactor) stays parked under the parent spec.
 
 Lifted from the broader codex runtime spec ([[file:docs/design/2026-05-28-generic-agent-runtime-spec.org]]) as the immediate-correctness slice independent of the larger arc. The singleton =.ai/session-context.org= is unsafe under simultaneous agents — two LLMs running in the same project at the same time would overwrite each other's session state.