Add read-only Forge session MCP server

[eshulman2]

Summary

• add a read-only Forge session summary service backed by checkpoint state
• expose production HTTP endpoint: GET /api/v1/sessions/{ticket_key}/summary
• expose an optional user-facing stdio MCP server with get_session_summary and forge://sessions/{ticket_key}
• keep forge-session-mcp out of the checked-in mcp-servers.json so Forge agents do not load session-inspection tools themselves
• add GRAFANA_BASE_URL config so summaries can include dashboard links
• add user docs for the HTTP endpoint and optional Claude/MCP setup

Safety

• users can query Forge API or their own assistant MCP client instead of needing Redis, Langfuse, or Grafana credentials
• responses intentionally omit raw prompts, model messages, generated artifacts, tool inputs, and raw trace context
• HTTP and MCP defaults expose checkpoint-derived summary only; Redis logs are opt-in on the HTTP endpoint and bounded by logs_limit <= 50
• the MCP server is configured by users in their assistant client, not by Forge's agent MCP configuration

Docs

• document /api/v1/sessions/{ticket_key}/summary in the API reference
• add docs/reference/session-inspection.md with HTTP and Claude MCP setup instructions
• link session inspection from README, docs index, and configuration reference

Tests

• jq empty mcp-servers.json
• uv run ruff check src/forge/mcp src/forge/api/routes/sessions.py src/forge/sessions tests/unit/mcp tests/unit/api/routes/test_sessions.py tests/unit/sessions/test_summary.py
• uv run pytest tests/unit/mcp/test_session_server.py tests/unit/api/routes/test_sessions.py tests/unit/sessions/test_summary.py