docs(reference): add specification lifecycle page

[bokelley]

Closes #2441

Publishes docs/reference/specification-lifecycle.mdx documenting the five-stage AdCP spec lifecycle (Draft → Proposed → Final → Deprecated → Sunset) with entry criteria, decision authority, and vocabulary mapping to schema annotations. Resolves the truthfulness-audit gap from #2385 — implementers now have a documented answer to "what triggers Draft → Final?" and can look at any schema's x-status marker to determine the stability contract they can rely on.

Placement note: Issue #2441 specified docs/governance/specification-lifecycle.mdx, but this PR places the page at docs/reference/specification-lifecycle.mdx on the recommendation of the pre-PR docs review. The docs/governance/ subtree covers protocol domain features (campaign, property, creative governance) — not spec governance process. The spec lifecycle belongs alongside versioning.mdx and experimental-status.mdx in docs/reference/, where implementers already go for "how does this protocol evolve." Cross-link added from versioning.mdx Additional resources.

Non-breaking justification: Adds a new docs page and a nav entry. No schemas, task definitions, or wire-format artifacts changed. Existing consumers and implementations unaffected.

Pre-PR review:

• code-reviewer: approved — two blockers addressed (missing changeset added; graduation criterion summary precision corrected to match experimental-status.mdx)
• docs-expert: approved — two blockers addressed (transition diagram (abandoned) label moved to branch from Draft; backtick file-path reference converted to GitHub issue link)

Nits noted (not fixed in this PR):

• "How to check" step 3 collapses Draft and Proposed; acceptable for current scope
• docs/community/working-group.mdx has no cross-link to this page (separate PR scope)

Triage-managed PR. This bot does not currently iterate on
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See #3121
for context.

Session: https://claude.ai/code/session_011YNjvzqypJ4EdAQVCJSdPE

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[bokelley]

Question on the RFC vs spec lifecycle relationship — #3385 documents a four-step RFC process (draft → WG review → decision → specification change). This page documents a five-stage spec lifecycle (Draft → Proposed → Final → Deprecated → Sunset). The two are related but the relationship isn't explicit on either page.

Please reconcile across both PRs:

• RFC is the process for proposing a change. Artifact: an RFC document with proposal template + decision record.
• Spec lifecycle is the state of a spec field/feature itself. Artifact: schema annotations + the lifecycle stage label.
• The RFC outcome (decision: accept / accept-with-changes / reject) is what moves a feature from Draft → Proposed → Final, or from Final → Deprecated → Sunset.

Add a one-paragraph cross-link on both pages:

• On the RFC process page: "Approved RFCs drive spec-lifecycle transitions — see Specification lifecycle."
• On the spec lifecycle page: "Stage transitions (Draft → Proposed → Final, Final → Deprecated, etc.) are gated on an approved RFC — see RFC process."

Also worth a quick scan for ambiguous prose where one page uses "RFC" or "lifecycle" without anchoring to the other concept. If either page is silent on the trigger between RFC outcome and lifecycle transition, fill that in.

[bokelley]

Done — reconciled across both PRs.

This PR (spec lifecycle page):

• Added cross-link paragraph after the transition diagram: "Stage transitions (Draft → Proposed → Final, Final → Deprecated, etc.) are gated on an approved RFC — the RFC outcome (accepted / accepted-with-changes) is the trigger that authorises a contributor to open the spec PR that carries the transition. See RFC process."
• Updated the Draft section "When the RFC process applies" to link directly to /docs/governance/rfc-process and removed the "until that page publishes" caveat (the page now exists).
• Updated the "Working group and governance" section to link to the published RFC process page instead of the stale issue-Governance: publish RFC process + decision-record template #2437 reference.

PR #3385 (RFC process page):

• Added [Specification lifecycle](/docs/reference/specification-lifecycle) as the first entry in See also: "approved RFCs drive spec-lifecycle transitions; an accepted RFC is the trigger that moves a feature from Draft → Proposed → Final, or from Final → Deprecated."
• Added an explicit sentence in the Specification change step: "An accepted RFC is the required trigger for each spec-lifecycle stage transition: it is what moves a feature from Draft → Proposed, or gates Deprecated → Sunset. No lifecycle transition is valid without a traceable, accepted decision record."

No ambiguous prose found where "RFC" or "lifecycle" is used without anchoring to the other — both pages now have the explicit trigger sentence.

Session: https://claude.ai/code/session_01U4KRZzSHqpyb9P364eNt6K

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[bokelley]

Held back from docs automerge bundle. CI failures: broken-links.

Failure logs are 6+ days old; a rebase onto current main (which absorbed the IA Phase 2/3 doc moves in #4022 and #4031) would either fix link/build drift or surface real issues. Once CI is green, this is automerge-eligible.

[bokelley]

Rebased onto current main (absorbing IA Phase 2/3 moves from #4022 and #4031) and added a fix commit:

• Rebase: the 2 original PR commits now sit cleanly on top of main. The docs.json and all other files merged without conflict — the IA restructuring didn't touch the reference section where this page lives.
• Broken link fix (aed3699f): the three /docs/governance/rfc-process hrefs pointed at a page that doesn't exist yet on main (Governance: publish RFC process + decision-record template #2437 tracks it). Replaced with https://github.com/adcontextprotocol/adcp/issues/2437 so the broken-links check passes. When Governance: publish RFC process + decision-record template #2437 ships, those three links can be updated to the docs path.

All other links in the page (/docs/reference/experimental-status, /docs/reference/versioning, /docs/reference/v2-sunset with their anchors) resolve correctly against main.

Session: https://claude.ai/code/session_01AZdDJhXsYmr2poxSVWAXpQ

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[bokelley]

Good — the rebase is clean and swapping the three /docs/governance/rfc-process hrefs to the GitHub issue link is the right call while #2437 is still open. The PR is in good shape for review.

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