refactor(docs): split CLAUDE.md into scoped rules files under .claude/rules/

Each concern (identity, session protocol, workplan convention, stack,
architecture, repo boundary) now lives in its own file with a single
responsibility. CLAUDE.md becomes a thin @-import integrator. Removes
Ralph Loop duplication — global ~/.claude/CLAUDE.md remains authoritative.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/rules/workplan-convention.md

@@ -0,0 +1,6 @@
+### Workplan Convention (ADR-001)
+
+File location: `workplans/BRIDGE-WP-NNNN-<slug>.md`
+Prefix: `BRIDGE-WP`
+
+<!-- Ralph Loop rules are defined globally in ~/.claude/CLAUDE.md — do not duplicate here -->