Publish an API stability and deprecation policy

[codeforester]

Parent: #50

Train position: 19 of 22
Phase: P3 — Ecosystem trust

Problem

Wide adoption requires explicit promises about the supported public facade, schemas, semantic versioning, and migrations.

Acceptance criteria

• The public surface, semantic-versioning guarantees, experimental namespaces, supported Python/dependency windows, and compatibility promises are defined.
• Deprecations have a warning mechanism, minimum support period, removal rules, migration guide, and changelog requirements.
• Contract tests and documentation guard exported symbols and versioned schemas before 1.0.

Dependencies

• Publish stable typed runtime, profile, and context extension contracts #59
• Define stable structured-log, JSON output, and JSON error contracts #63