ADR-0087 P3: MCP upgrade surface + RC rehearsal discipline

[os-zhuang]

Part of #2643 (ADR-0087). The AI-native consumption channel for the D4 artifacts, plus the rehearsal topology (D6). Deferred behind P0–P2 and evidence-gated per the ADR (built when there are enough external consumers to justify the surface).

Depends on P2 (the MCP tools serve the change manifest + migration set).

Scope

D4 — MCP upgrade tools (@objectstack/mcp)

• spec_changes(from, to) — any version pair, folded across majors from the composable per-major spec-changes.json.
• spec_deprecations(stackPath) — which conversions my metadata currently triggers.
• spec_migrate(dryRun) — preview / apply the chain.

A consumer-side agent asks "what breaks me between 10 and 14?" and gets data, not documentation.

D6 — RC rehearsal (topology inverts)

• Every major is published to the next dist-tag at least one RC cycle before latest.
• Document the consumer-pull model: a consumer-side agent subscribes to the RC, runs its own validate && typecheck && test (the P0 handshake → P1/P2 migrate → verify loop) in a branch, and opens its own migration PR. The framework only publishes the RC + D4 artifacts — it never executes other repos' CI.
• The pre-publish hotcrm smoke stays (release-blocking, unchanged) as the framework-side floor. A framework-side "community smoke ring" is explicitly rejected (wrong direction — don't scale framework-side execution when consumers can execute themselves).

Acceptance criteria

• The three MCP tools answer from spec-changes.json for an arbitrary from/to pair spanning multiple majors, with no separate documentation lookup.
• A documented, reproducible RC-consumer loop exists (an agent can rehearse an upgrade against next before latest).
• The Release workflow publishes next before latest for a major; hotcrm smoke remains the only release-blocking external gate.

Boundaries

• The consumer-side autonomous-upgrade agent itself is out of scope (consumers / ADR-0033 toolchain own it); P3 makes it possible by exposing the surface.
• Evidence gate: build the MCP tools + RC step when external-consumer demand justifies the operational surface (ADR-0087 §Boundaries, like ADR-0054 Phase 3).

Refs: #2643, ADR-0087 (D4 tools, D6), ADR-0025, ADR-0033.