docs: update ARCHITECTURE + CONTRIBUTING for new placeholders

Add {{BASE_BRANCH_DETECT}} to ARCHITECTURE.md placeholder list.
Cross-reference CLAUDE.md template authoring guidance from CONTRIBUTING.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ARCHITECTURE.md

@@ -179,7 +179,7 @@ gen-skill-docs.ts      (reads source code metadata)
 SKILL.md               (committed, auto-generated sections)
 ```
 
-Templates contain the workflows, tips, and examples that require human judgment. The `{{COMMAND_REFERENCE}}` and `{{SNAPSHOT_FLAGS}}` placeholders are filled from `commands.ts` and `snapshot.ts` at build time. This is structurally sound — if a command exists in code, it appears in docs. If it doesn't exist, it can't appear.
+Templates contain the workflows, tips, and examples that require human judgment. Placeholders like `{{COMMAND_REFERENCE}}`, `{{SNAPSHOT_FLAGS}}`, `{{UPDATE_CHECK}}`, `{{BROWSE_SETUP}}`, and `{{BASE_BRANCH_DETECT}}` are filled from `commands.ts`, `snapshot.ts`, and `gen-skill-docs.ts` at build time. `{{BASE_BRANCH_DETECT}}` provides dynamic base branch detection for PR-targeting skills (ship, review, qa, plan-ceo-review) so they work with stacked branches and non-main targets.
 
 ### Why committed, not generated at runtime?
 

CONTRIBUTING.md

@@ -180,6 +180,8 @@ bun run skill:check
 bun run dev:skill
 ```
 
+For template authoring best practices (natural language over bash-isms, dynamic branch detection, `{{BASE_BRANCH_DETECT}}` usage), see CLAUDE.md's "Writing SKILL templates" section.
+
 To add a browse command, add it to `browse/src/commands.ts`. To add a snapshot flag, add it to `SNAPSHOT_FLAGS` in `browse/src/snapshot.ts`. Then rebuild.
 
 ## Conductor workspaces