chore(gitignore): extend planning-scratch exclusions to cover session-scoped notes

Mirrors the existing v14*-/QA-/round- patterns. Local audit and survey
artefacts written under `.planning/marathon-*`, `.planning/security-audit-*`,
`.planning/session-*`, and any future v15-stream scratch never need to
leave the working tree.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -76,9 +76,13 @@ next-env.d.ts
 /.planning/QA-*
 /.planning/round-*
 /.planning/v14*-*
+/.planning/v15*-*
 /.planning/V0*-*
 /.planning/ios-coord/V*-QA-*
 /.planning/research/v14*-*
+/.planning/marathon-*
+/.planning/security-audit-*
+/.planning/session-*
 
 # Docs scratch — temporary spec drafts that live in the repo only
 # while a release is in flight.