feat: inject project .dag as ground truth before markdown context

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Problem

Agent reads .ts implementation files before .dag spec files — bypasses DAG-first engine entirely. Observed in magic-cabinet-mvp session where agent read 8+ .ts files before any .dag, requiring 4 user interventions.

Root cause

Project .dag files are not pre-injected into context. Agent must explicitly read_file them — same cost as .ts files, but less obviously useful. The .dag has no code pointers, so it doesn't serve as a natural first stop.

Fix

• prompt_builder.py: _load_project_dag() scans projects/ and specs/ for .dag files, compacts JSON to DAG-path notation (6435→1046 chars, 16%), injects as turn-1 context BEFORE any markdown. Priority: .dag → .dag.md → AGENTS.md → CLAUDE.md → .cursorrules.
• subdirectory_hints.py: *.dag glob added to hint discovery. When agent navigates to directories with .dag files, they're compacted and injected as [Subdirectory DAG context] blocks.

Before/After

Before: Agent starts session, sees AGENTS.md markdown, then reads .ts files, never touches .dag unless told.

After: Agent wakes up seeing:

[contractor-dashboard]  # DAG ground truth
Contractor→ Project:owns(1:N) | Order:places(1:N) | ...
  states: unregistered → registered → verified → pro
Design2020Import→ Project:imports_via(1:N) | DesignVersion:produces(1:1)
  states: uploaded → parsed → matched → rendered → priced

Gap detection is immediate: DAG says priced state exists, implementation has matchedSku: null.