bootstrap: implement repo-local artifact inventory and classification

[flyingrobots]

Summary

Implement the first repo-local artifact inventory pass for semantic bootstrap.

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

Scope

Discover and classify:

• source files
• markdown docs
• ADR files
• key project files
• repo-local references to issues / PRs / commits

Acceptance Criteria

• inventory boundaries and defaults are deterministic
• artifact classes are explicit and inspectable
• the scanner is useful on an unfamiliar repository without external APIs

Related

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