Deprecation channel: policy in RELEASING.md + runtime warnings

[mchmarny]

Summary

Define the deprecation channel for all four frozen surfaces, document it in
RELEASING.md, implement the runtime warning mechanism, and exercise it once
before v1.0.0.

Why v1

ROADMAP §2
shippable artifact:

Deprecation channel documented in RELEASING.md.

and demonstrable signal:

Deprecation channel exercised at least once in practice before tagging v1.0.0.

The second one is a scheduling constraint, not just a doc task — a real
deprecation has to ship in a pre-v1.0.0 release with a working warning.

Current state

RELEASING.md §Versioning is three bullets: semver, RC naming, and "breaking
changes increment MAJOR." There is no statement of what "breaking" means per
surface, no notice period, and no mechanism for warning a user before removal.

Scope

1. Policy (RELEASING.md). For each of CLI, REST, Go SDK, and bundle/schemas:
what counts as breaking, what counts as additive, the minimum notice period in
releases or calendar time, and where deprecations are announced.

2. Mechanism, per surface:

• CLI — deprecated flags/commands still work but emit a warning to stderr
  naming the replacement and the removal release. Must respect NO_COLOR and
  the existing logger conventions.
• REST — Deprecation and Sunset response headers (RFC 8594) on
  deprecated endpoints, plus deprecated: true in api/aicr/v1/server.yaml.
• Go SDK — // Deprecated: godoc markers, which staticcheck surfaces to
  consumers automatically.
• Schemas/bundle — deprecated fields marked in the generated JSON Schema
  and honored-but-warned by the loader.

3. Announcement surface — a ## Deprecations section in release notes and
a durable page under docs/user/.

4. Exercise it once — pick a genuine deprecation and ship it through the
full channel pre-v1.0.0. The /v1 vs /v2 REST path families (#2112) are a
natural candidate if /v1/* is being retired.

Acceptance criteria

• Per-surface breaking/additive definitions and notice period in RELEASING.md
• CLI deprecation warning implemented and tested
• REST Deprecation/Sunset headers + deprecated: true in the OpenAPI spec
• // Deprecated: convention documented for pkg/client/v1
• Deprecations section in release notes; durable page in docs/user/
• At least one real deprecation shipped through the channel before v1.0.0

Related

#2111, #2112, #2113 (the gates that will point users at this channel when they
trip), #2114 (apiVersion promotion is itself a candidate first exercise).

[mchmarny]

Triaged: linked as a sub-issue of epic #2370 (API freeze — baseline and CI-gate every v1 surface). No board field changes.

Rationale: this issue was an orphan, so the four ROADMAP §2 surfaces had no shared parent and no visible sequencing. #2370 collects them, records which surface is already gated (the Go SDK, via #2018 under epic #2016), and states the dependency order — #2114 before #2113, and #2115 after all four surfaces are defined.