feat(authoring): add accelerated Milestone A E2E gate

[777genius]

The standard-first authoring stack had strong synthetic contracts but no small, exact-candidate gate for the public user journey. This PR adds the accelerated Milestone A checkpoint on top of #216: both public entrypoints build from the PR head, run in isolated disposable roots, and prove equivalent authoring and installer dry-run results without using real projects.

The new gate runs the full Linux journey through init, validate, inspect, static test, and local agentplugins add --dry-run, plus packaged launcher/init/validate smoke on Windows amd64 and macOS arm64. Candidate packages come only from locally packed exact-head tarballs. Production installer security remains enabled and may read its credential-free pinned public feeds. The workflow contains no publication path.

This also closes the deferred public-packed dispatch boundary found during independent review: foreign, missing, malformed, duplicate-key, noncanonical, and semantically invalid selected-source inputs reject before checkout or provisioning. Existing YAML capability code remains preserved.

Validation before push:

• Independent exact-tree review: ACCEPT, no P0/P1/P2 findings.
• Public workflow boundary: 6/6 passed.
• Milestone A executable boundary: 16/16 passed.
• Exact integrated source diff SHA-256: 0b18d2540e458b2ae3ff2cb33bee6e6148b9e4c12401e10770a3453752842097 before the reviewed plan-only commit.
• Genuine GitHub-hosted matrix and required repository checks are expected on this PR head.

Stack dependency: #216

Refs #216
Refs #208

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