TDD with commit trailers

type:feat and type:fix issues ship a failing behavior test first, then the implementation. Enforcement uses commit trailers, not git-log topology (which breaks on squash-merges, rebases, and legitimate single-commit fixes).

The TDD tier rubric

Not every change deserves a red→green dance:

Tier	Applies when	Required artifact
tdd-tier:strict	Default for type:feat and type:fix that change behavior	A test(...) commit and a Tdd-Red: <sha> trailer on the green commit
tdd-tier:smoke	One-line variants, CSS-only tweaks, prop additions with default	A ?raw source-text or render-once assertion is enough
tdd-tier:exempt	Copy fixes, comments, ADR additions, dep bumps with no behavior change	No new test required

prometeo proposes the tier; the human accepts or edits before the run starts. centinela enforces against the labelled tier, not a global rule.

The trailer convention

The green commit carries a Git trailer:

feat(ui): button supports loading state (#N)

Adds a `loading` prop that disables clicks and shows a spinner.

Tdd-Red: 0a1b2c3

Tdd-Red: <sha> points back to the originating red commit on the branch (survives rebase, survives squash because the trailer is preserved in the merge commit message). For legitimate single-commit work (regression test+fix in one), use Tdd-Red-Verified: inline.

The loop

1. prometeo emits a ## Behavior contracts section per issue: 1-3 user-observable behaviors that must hold.
2. forja writes the failing test, commits it (test(scope): red for <behavior> (#N)), runs vitest --run to confirm red, writes the implementation, commits it with the Tdd-Red: trailer.
3. centinela greps the green commit for the trailer and re-runs the red commit’s test to confirm it failed.

Source-text ?raw assertions stay as smoke tests but don’t count as a strict-tier behavior contract.

Bugs found in production: regression test first, fix second — non-negotiable regardless of tier.

Full rationale → docs/PRINCIPLES.md §2