compiler: show full include chain for import-cycle errors

[davidahmann]

Operators lose time debugging import cycles because compile errors currently show only a pair of files instead of the full include/import chain that triggered the cycle.

Why now:

• Nested import usage has increased.
• We are seeing recurring triage friction when cycle depth is >2.

Expected behavior:

• Import-cycle failures report a deterministic full chain (including the first back-edge) so operators can fix the cycle in one pass.

Evidence packet:

• Commit under test: origin/main @ 1f71659
• Runtime: Go 1.25.7, Darwin arm64 (macOS 26 kernel 25.3.0)
• Minimal repro:
  1. Create 4 workflows with imports A->B->C->D->B
  2. Run gh aw compile on A
  3. Observe error text
• Expected: deterministic chain with back-edge path.
• Actual: partial cycle context (insufficient path detail).

Acceptance / validation:

• Unit test for a 4-file cycle verifies full deterministic chain output.
• Error still fails closed and remains deterministic across runs.

[pelikhan]

We used to generate a full mermaid graph in the header

[pelikhan]

The error message should be delightful multiline indented agent friendly.