bootstrap: implement first-pass entity extraction for files docs ADRs and modules

[flyingrobots]

Summary

Implement first-pass entity extraction for the Hill 1 bootstrap slice.

This is Slice C from the Hill 1 semantic bootstrap spec.

Scope

Create conservative extraction for:

• file: nodes
• doc: nodes
• adr: nodes
• module: nodes when structure is obvious

Acceptance Criteria

• extracted entity types are stable and conservative
• module: inference does not invent fragile structure where evidence is weak
• extracted entities are persisted in the graph with enough metadata for follow-on inference

Related

• bootstrap: define first semantic bootstrap slice for unfamiliar repositories #303
• docs/design/h1-semantic-bootstrap.md