fix(mcp): actionable error when graph.db is missing

[smochan]

Summary

• New `GraphNotBuiltError` raised by `_load_graph` when `.codegraph/graph.db` is absent.
• MCP dispatcher catches it and returns `{error: "graph_not_built", message: "...Run \`codegraph build\`...", db_path: ...}`.
• Replaces the silent "empty result" behaviour that LLMs mistranslated as "workspace is empty".

v0.1.2 backlog item #8.

Test plan

• 2 new tests in `tests/test_mcp_graph_not_built.py` (raise on missing db; dispatcher returns structured error).
• Existing MCP suite (20 tests) green.

🤖 Generated with Claude Code

[github-actions]

codegraph PR review

Diff vs main · severity ≤ high

codegraph review (target: main)

Diff: +8 / -0 / ~0 nodes, +28 / -0 edges

Findings (1)

severity	rule	qualname	file:line	score	message
low	new-dead-code	codegraph.mcp_server.server.GraphNotBuiltError.__init__	codegraph/mcp_server/server.py:28	10	Potentially unreachable new code

_{Triggered by codegraph CI · last run}

[github-actions]

codegraph PR review

Diff vs main · severity ≤ high

codegraph review (target: main)

Diff: +75 / -0 / ~0 nodes, +301 / -1 edges

Findings (1)

severity	rule	qualname	file:line	score	message
low	new-dead-code	codegraph.mcp_server.server.GraphNotBuiltError.__init__	codegraph/mcp_server/server.py:28	10	Potentially unreachable new code

_{Triggered by codegraph CI · last run}

[github-actions]

codegraph PR review

Diff vs main · severity ≤ high

codegraph review (target: main)

Diff: +8 / -0 / ~0 nodes, +28 / -0 edges

Findings (1)

severity	rule	qualname	file:line	score	message
low	new-dead-code	codegraph.mcp_server.server.GraphNotBuiltError.__init__	codegraph/mcp_server/server.py:28	10	Potentially unreachable new code

_{Triggered by codegraph CI · last run}