docs(shared): add sdd-gates fragment (Unit 7 prerequisite)

[norrietaylor]

Summary

Adds shared/sdd-gates.md, the new importable gh-aw fragment for Unit 7
(the sdd-validate agent). This is PR-A of the two-PR pattern Unit 4 used:
the fragment must be on main before sdd-validate.md can import it via the
pinned-ref norrietaylor/spectacles/shared/sdd-gates.md@main form, since
gh aw compile resolves @main imports by fetching from GitHub.

What it contains (R7.2)

The fragment enumerates the four per-boundary validation gate sets:

• Spec gates — acceptance criteria testable, no implementation leakage,
  assumptions explicit, proof artifacts present and behavioral.
• Architecture gates — a decision and rationale present, alternatives
  considered, consistent with existing decisions/, no implementation detail
  masquerading as a decision.
• Triage gates — every spec R-ID covered by a task, dependencies form a
  DAG, each task single-session sized, every task has a repo: field.
• Implementation gates — proof artifacts re-executed and passing, changed
  files within task scope, no real credentials in the diff.

It also defines the Blocker / Warning / Info severity model: a Blocker
triggers the needs-human hand-off, consistent with ADR 0001 and the
advisory-by-design stance of the spec.

Notes

• markdownlint clean; no em dashes; ATX headings; no private literal.
• Layout per ADR 0002: the fragment stays at repo-root shared/.
• PR-B (the sdd-validate agent that imports this) follows once this is
  merged.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 84c63b5f-b067-4287-807e-33271d4ef968

📥 Commits

Reviewing files that changed from the base of the PR and between a74aee0 and 1acaea5.

📒 Files selected for processing (1)

• shared/sdd-gates.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch sdd/unit-7-gates

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[norrietaylor]

Verdict: Approve-equivalent. shared/sdd-gates.md is complete against R7.2, accurate, and consistent with the existing shared/*.md fragments. No blocking findings.

Reviewed against spec R7.2 (the four per-boundary gate sets), repo standards, and consistency with sibling fragments. Findings: 0 Blocker, 0 Major, 0 Minor, 2 Nit.

R7.2 completeness — pass

All four named gate sets are present and each enumerates the exact sub-items R7.2 requires:

• Spec gates (lines 26-40): acceptance criteria testable, no implementation leakage, assumptions explicit, proof artifacts present and behavioral. All four present.
• Architecture gates (lines 42-58): decision and rationale present, alternatives considered, consistent with existing decisions/, no implementation detail masquerading as a decision. All four present.
• Triage gates (lines 60-75): every spec R-ID covered by a task, dependencies form a DAG, each task single-session sized, every task has a repo: field. All four present.
• Implementation gates (lines 77-89): proof artifacts re-executed and passing, changed files within task scope, no real credentials in the diff. All three present.

The proof artifact "File: shared/sdd-gates.md contains all four named gate sets" is satisfied.

Accuracy and consistency — pass

• Severity model (Blocker / Warning / Info, lines 14-24) matches R7.3, and the "Blocker triggers needs-human, never a failed required check" stance matches R7.4 and ADR 0001. Correct.
• Boundary descriptions (*-spec-*.md, architecture.md/decisions/**, sdd:ready label event, other PR changes) match R7.1's boundary-resolution rules.
• ## Verification section present, consistent with sdd-interaction.md and sdd-proof-artifacts.md.
• No em dashes, ATX headings, no private literals, fenced code where applicable. Repo standards met.

Nits (non-blocking)

1. shared/sdd-gates.md:81-86 — implementation gate 2 bundles two distinct checks under one gate. "Changed files within task scope" assigns Warning to an out-of-scope change and Blocker to a protected-path change. R7.2 names this as a single gate so this is acceptable, but a reader scanning gate-to-severity mapping sees one numbered gate carrying two severities. Optional: split protected-path into its own numbered gate, or add a one-line note that this gate has two severity tiers.

2. PR description uses em dashes (e.g. "Spec gates — acceptance criteria..."). The file itself is clean, which is what the repo standard governs, but the PR body is a public artifact too; worth keeping consistent. Not a code defect.

Net: this fragment is ready to merge as the Unit 7 prerequisite. sdd-validate.md (PR-B) can import it via the pinned-ref @main form once this lands.