docs(adr): add ADR-0002 command contract validation module

docs/adr/0002-command-contract-validation-module.md

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+# Command Contract Validation Module
+
+We decided to centralize the `commands/gsd/*.md` file contract into a single validation seam enforced at two layers: a fast lint script (`scripts/lint-command-contract.cjs`) that runs as a pre-test CI step, and a behavioral regression test (`tests/command-contract.test.cjs`) that validates the full contract against the live filesystem.
+
+## Decision
+
+The command file contract defines what makes a valid `commands/gsd/*.md`:
+
+- `name:` field present, non-empty, matches `gsd:*` or `gsd-*` (ns- commands use `gsd-`)
+- `description:` field present and non-empty
+- `allowed-tools:` block present and non-empty, all entries from the canonical tool set
+- Every `@`-reference inside `<execution_context>` blocks resolves to an existing file on disk
+- `@`-references inside `<execution_context>` blocks appear on their own line (no trailing prose)
+
+## Context
+
+Before this ADR, the command contract was enforced inconsistently:
+- `tests/enh-2790-skill-consolidation.test.cjs` checked existence and frontmatter of specific post-consolidation commands
+- `tests/bug-3135-capture-backlog-workflow.test.cjs` checked `execution_context` @-ref resolution (added 2026-05-05)
+- No test checked `allowed-tools` validity, `name:` convention, or `description:` non-emptiness across all commands simultaneously
+
+This meant any PR touching a command file could break the contract without a single test catching it. The `add-backlog.md` gap (#3135) is a concrete example: the workflow file was missing for the full consolidation cycle before a targeted regression test was written.
+
+Additionally, 40 of 65 command files contained redundant prose @-references — the same path appearing once in `<execution_context>` (which loads the file) and again in `<process>` body text (inert). This added ~900 tokens of dead weight per invocation and created a drift seam where prose refs could go stale independently of the executable `execution_context` ref.
+
+The two largest commands (`debug.md`, `thread.md`) embedded their full implementation inline rather than delegating to workflow files, causing ~4,400 tokens of implementation detail to load as part of the skills index description on every session regardless of whether those commands are used.
+
+## Consequences
+
+- A single `lint-command-contract.cjs` script enforces frontmatter invariants across all 65 commands in milliseconds, runs before the test suite in CI
+- `tests/command-contract.test.cjs` replaces the scattered contract coverage in `enh-2790` and `bug-3135`, becoming the authoritative behavioral contract test for the entire command surface
+- Redundant prose @-refs removed from 40 command files (~900 tokens/invocation recovered)
+- `debug.md` and `thread.md` refactored to the workflow-delegation pattern (~4,400 tokens removed from eager system-prompt load)
+- `workflows/extract_learnings.md` renamed to `workflows/extract-learnings.md` to align with the hyphen convention used by all other workflow files
+- The `execution_context` block is the single authoritative declaration of what a command loads — no duplication in prose