Clean up remaining get-session-context migration loose ends

[williamthorsen]

Context

Follow-up from #319 (rename get-branch-context to get-session-context). Three items were identified during review but deferred as below the approval threshold.

Items

1. Update stale get-ticket-id reference in artifact-conventions.md

File: packages/agents/content/skills/_data/artifact-conventions.md:76

Line 76 says "Use get-ticket-id for the ticket ID" while the path resolution section directly above (line 60) was updated to direct callers to use get-session-context. The get-ticket-id skill still exists and the reference is not broken, but it is inconsistent with the new canonical guidance.

Update line 76 to read "Use get-session-context to obtain ticket_id" or remove the guidance sentence if the path template above already makes it self-evident.

2. Remove unnecessary get-session-context dependency from aspect reviewer subagents

Files:

• packages/agents/content/subagents/aspect-code-reviewer.md
• packages/agents/content/subagents/aspect-silent-failure-reviewer.md
• packages/agents/content/subagents/aspect-test-reviewer.md

These subagents previously listed get-default-branch in their skill dependencies and now list get-session-context. In orchestrated contexts, the orchestrator already passes merge-base-sha directly, so the subagents never invoke get-session-context. In fallback paths, the full manifest derivation (preferences read + file write) is wasted work for ephemeral subagents.

Remove the get-session-context skill dependency entirely — no replacement is needed since the orchestrator passes all required parameters in the Task prompt.

3. Remove unnecessary get-session-context dependency from orchestrated-reviewer

File: packages/agents/content/subagents/orchestrated-reviewer.md

The orchestrated-reviewer lists get-session-context as a skill dependency and documents a fallback path in process step 1: "If none was provided, compute it yourself: invoke get-session-context, then run git merge-base HEAD <default-branch> to get the SHA."

This fallback guards against a scenario that cannot occur: the orchestrator always computes {merge-base-sha} before entering the review cycle (it is a required input variable per review-cycle.md), and the orchestrated-reviewer is only dispatched from the orchestrated pipeline. Both dispatch sites (Phase 4 core review and Phase 4b holistic review) pass the SHA explicitly.

Remove get-session-context from the skills list and replace the fallback language with a direct instruction to use the provided SHA.