bootstrap: implement first-pass relationship inference with provenance and confidence

[flyingrobots]

Summary

Implement the first-pass relationship inference layer for semantic bootstrap.

This covers Slices D and E from the Hill 1 semantic bootstrap spec.

Scope

Infer a small initial set of relationship types:

• documents
• references
• touches
• groups
• conservative implements

And attach:

• provenance metadata
• rule-based confidence bands
• enough surfaced information to review weak-confidence output

Acceptance Criteria

• each inferred assertion includes inspectable provenance
• confidence is rule-based and legible
• low-confidence edges are clearly marked
• the first slice improves repo understanding without requiring manual graph authoring first

Related

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