feat: signal-driven persona selection, constraints, handoffs, and orchestration

[robinojw]

Summary

Inspired by opensoft/oh-my-opencode's multi-agent architecture, this PR makes roster's personas smarter, more actionable, and self-orchestrating.

Changes

Signal-driven persona selection (selector.go)

• Only relevant personas are now written to .roster/personas/ based on detected repo signals
• 5 core personas always included: architect, reviewer, test, docs, security
• Frontend personas (design, accessibility) added when TypeScript/JavaScript, design systems, Storybook, or frontend frameworks detected
• Conditional personas: devops (CI/Docker), api (web frameworks), data (data-heavy languages/frameworks), performance (frontend/E2E)
• A minimal Go CLI gets 5 personas instead of 11 — no more irrelevant design/accessibility advice

Richer persona definitions (all 11 .md files)

• role field (planning / execution / review) — enables hierarchical orchestration patterns
• ## Constraints section — explicit anti-patterns and "must not" rules per persona, preventing common failure modes
• ## Handoffs section — defines upstream/downstream relationships between personas for delegation and verification flows

Enhanced managed section (managed.go)

• Codebase Context Hints — detected signals rendered as structured hints (languages, frameworks, test tools, CI, linter, etc.) to streamline persona hydration
• Orchestration Rules — trigger-keyword-to-persona routing table with role-based interaction patterns (planning → execution → review)

Pipeline changes (writer.go)

• WriteAll now selects personas via Select() before writing, so only relevant persona files and table entries are emitted

Testing

• 10 new selector tests covering: core always-included, Go-only exclusions, CI/Docker triggers, React/Tailwind frontend, Django data+api, Python data, full-stack all-11, minimal 5-core
• New TestPersonaRoleIsValid ensures all personas have valid role values
• Updated integration test expectations for signal-based counts
• All tests pass with -race, golangci-lint run reports 0 issues