feat: add proactive session-start coaching

[ccf]

Summary

• add a session-start coaching brief endpoint that reuses profile, playbook, and project signals
• expose a dedicated MCP tool for contextual coaching at the start of a session
• mark the roadmap item for proactive session-start coaching as shipped

Testing

• PYTHONPATH=src pytest tests/test_coaching.py tests/test_mcp_tools.py tests/test_mcp_server.py -q
• ruff check src/primer/common/schemas.py src/primer/server/services/coaching_service.py src/primer/server/routers/analytics.py src/primer/mcp/tools.py src/primer/mcp/server.py tests/test_coaching.py tests/test_mcp_tools.py tests/test_mcp_server.py
• ruff format --check src/primer/common/schemas.py src/primer/server/services/coaching_service.py src/primer/server/routers/analytics.py src/primer/mcp/tools.py src/primer/mcp/server.py tests/test_coaching.py tests/test_mcp_tools.py tests/test_mcp_server.py

---

Note

Medium Risk
Adds a new analytics endpoint and MCP surface that synthesizes multiple data sources (profile + project workspace) into guidance; risk is moderate due to new branching/selection logic and new response fields that clients must handle.

Overview
Adds proactive session-start coaching alongside the existing retrospective coaching flow.

Introduces GET /api/v1/analytics/coaching/session-start plus a new MCP tool session_start_coaching(...) that accepts optional project_name, workflow_hint, and task_hint, and returns a contextual brief (starting pattern, recommended model/tools, and watch-outs) derived from engineer profile + project workspace signals.

Extends CoachingBrief with brief_type and optional context_summary, and refactors MCP rendering via _render_coaching_brief so both retrospective and session-start briefs share formatting; adds targeted tests covering auth gating, hint normalization, and MCP client behavior.

Marks the roadmap item for session-start coaching as shipped.

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