docs: add contribution guide and BMAD workflow

[stephenleo]

Summary

• Adds a contribution guide explaining CI checks and how to pass them
• Documents both the BMAD-assisted workflow and the manual workflow for adding new modules
• Adds guidance for updating documentation, showcase, and examples

Test plan

• Review guide content for accuracy against current CI setup
• Verify BMAD workflow steps reflect current skill names
• Confirm manual module workflow matches CLAUDE.md patterns

🤖 Generated with Claude Code

[claude]

Code review

Found 2 issues. See inline comments below.

---

Issue 1 — docs/contributing.md lines 40–42: "no other files required" contradicts the next bullet

Line 40 says — no other files required immediately before line 42 instructs contributors to also edit src/config.rs. A contributor reading this literally would skip the config step. Per CLAUDE.md L18–19, adding a module involves the module file + src/modules/mod.rs, but src/config.rs is also required when the module needs config fields (which every module does).

Issue 2 — CONTRIBUTING.md lines 42–44: same contradiction + mirror is out of sync

Same "no other files required" problem as above. Additionally, both files state they are kept in sync manually, but docs/contributing.md has an extra bullet that CONTRIBUTING.md is missing:

Absent data → explicit match + tracing::warn! + None. Disabled flag → silent None. (Exception: context_bar renders an empty bar with tracing::debug! when context data is absent — this is intentional UX.)

This rule is defined in CLAUDE.md L9 and should appear in both contributing guides.