#9221: fix(skills): use skillKey for env config lookup in snapshots

## Summary When skills have a custom `skillKey` in their metadata (e.g., `skillKey: 'trello'` for a skill named `trello-skill`), the env var config lookup in `applySkillEnvOverridesFromSnapshot` was failing because it used `skill.name` instead of the skillKey. This caused skill env vars defined in `skills.entries.<skill>.env` to not be injected into subagent exec commands when the skill uses a custom skillKey for the config. ## Changes - Add `skillKey` to `SkillSnapshot.skills` type (stored only when it differs from name) - Update `buildWorkspaceSkillSnapshot` to include skillKey in the snapshot - Update `applySkillEnvOverridesFromSnapshot` to use skillKey (with fallback to name) for config lookup - Add test case for skillKey-based env lookup ## Testing - All existing tests pass - New test case verifies skillKey-based env lookup works Fixes #9207 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR fixes a config lookup bug for skill environment overrides when a skill uses a custom `skillKey` in its frontmatter metadata. It updates the serialized `SkillSnapshot` shape to optionally include `skillKey` (only when it differs from `name`), updates snapshot building to store it, and updates `applySkillEnvOverridesFromSnapshot` to resolve config entries by `skillKey ?? name`. A new unit test covers the `skillKey`-based lookup path. This change integrates with the existing skills pipeline by affecting the snapshot used to avoid reloading skills during embedded runs (`src/agents/pi-embedded-runner/*`), ensuring env injection uses the same config keying as the non-snapshot path (`applySkillEnvOverrides`, which already resolves the skill key from frontmatter). <h3>Confidence Score: 4/5</h3> - This PR is close to safe to merge, but there is a concrete snapshot type mismatch that can break session snapshot persistence/compatibility and may drop the new `skillKey` field. - Core logic change is small and aligns snapshot env lookup with the existing non-snapshot codepath, and the new unit test covers the intended behavior. However, `SessionSkillSnapshot` in `src/config/sessions/types.ts` was not updated to include `skillKey`, which can cause type incompatibilities and prevent `skillKey` from being retained when snapshots are stored/loaded via session types. - src/config/sessions/types.ts <!-- greptile_other_comments_section --> <!-- /greptile_comment -->