feat: define Codex review contract and reject unsupported agents

[douglasjarquin]

Closes #4

Summary

• Adds a Made-owned, versioned Codex review prompt and strict structured-output contract with trusted base, candidate input/output identity, exact diff command, scope, taxonomy, exclusions, bounded task/output/execution, and review evidence.
• Requires explicit candidate-diff inspection, exact affected paths, controlled auto-fix policy, and fail-closed malformed, unknown-kind, missing-field, and invented-invocation rejection.
• Verifies trusted-base availability in a detached review copy and preserves read-only Codex execution, no-write/no-merge boundaries, and supported-agent truthfulness.
• Rejects unsupported agents before pipeline stages, removes Claude from capabilities and generated skill text, and resolves candidate output identity from the actual reviewed HEAD before evidence.

Validation

• Final source SHA: 84aec895a410b72331d06817125e0fc4ff1fcbee.
• Trusted base SHA: cdf0b40cc5a07790cb9d512b4b931b8df19145fa.
• GOTOOLCHAIN=local go build ./... passed.
• GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=commit.gpgsign GIT_CONFIG_VALUE_0=false GOTOOLCHAIN=local go test ./... passed.
• GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=commit.gpgsign GIT_CONFIG_VALUE_0=false GOTOOLCHAIN=local go test -race -shuffle=on -count=1 ./... passed.
• GOTOOLCHAIN=local go vet ./... passed.
• make lint passed with 0 issues.
• Repository LSP diagnostics reported 0 errors and 0 diagnostics.
• made capabilities --json reports only codex.

Native Codex smoke

• /opt/homebrew/bin/codex codex-cli 0.147.0 is authenticated with ChatGPT credentials.
• A disposable safe-to-unsafe repository was reviewed with codex exec --cd <review> --json --output-schema <schema> --sandbox read-only --ephemeral -.
• The bounded retry exited 0 with a schema-valid blocking finding naming check.sh and the exact affected path.
• Receipt: /tmp/made-issue4-final-receipt-84aec89.md.

Hosted delivery and custody

• PR feat: define Codex review contract and reject unsupported agents #21 is open, non-draft, unmerged, and against main.
• Hosted CI run 32157282348 passed at its exact recorded head 84aec895a410b72331d06817125e0fc4ff1fcbee.
• Hosted job build-test-lint is 95777423737 and passed.
• This task used the single persistent worktree and did not create another worktree, branch, child thread, merge, or auto-merge.
• Issue Harden reviewer filesystem isolation and enforce project-settings policy #5 was not started.
• This PR remains intentionally open for human review.