Prepare release 0.9.0

[hexsecs]

Summary

Release-prep only: bump version metadata to 0.9.0 and promote the CHANGELOG.md [Unreleased] section to a dated 0.9.0 release heading.

main is currently on 0.8.1.dev0 (the default next-patch target after 0.8.0). This release intentionally targets 0.9.0 (a minor bump) instead, per the documented override in docs/release.md.

Closes #487

Changes

• src/canarchy/__init__.py: __version__ set to 0.9.0.
• CHANGELOG.md: [Unreleased] entries promoted to ## [0.9.0] - 2026-07-23; a fresh empty [Unreleased] heading remains for future work.

Test plan

• uv run pytest -q — 1540 passed, 1 skipped, 99 subtests passed
• uv run ruff check — all checks passed
• uv build — sdist and wheel built for canarchy-0.9.0
• uvx twine check dist/* — both artifacts passed
• Installed the built wheel into a clean venv; canarchy --version reports canarchy 0.9.0
• uv run --group docs mkdocs build --strict — succeeded

Documentation

• CHANGELOG.md updated (Unreleased promoted to 0.9.0)
• docs/command_spec.md updated (if the command surface changed) — not needed, no command-surface change
• docs/event-schema.md updated (if the structured-output shape changed) — not needed, no event-shape change
• Touched docs/design/ spec updated (EARS syntax) — not needed, no behaviour change
• Touched docs/tests/ spec updated (Gherkin Given/When/Then) — not needed, no behaviour change
• AGENTS.md / docs/agents.md updated (if agent workflows changed) — not needed
• mkdocs.yml nav updated (if any new docs pages were added) — not needed

Safety

• Active-transmit commands require --ack-active — not applicable, no active-bus behaviour change
• SECURITY.md reviewed against the new behaviour — not applicable, no behaviour change

Next steps (not part of this PR)

Per docs/release.md, after this PR merges: tag v0.9.0, build/verify release artifacts from the tagged commit, publish to TestPyPI then PyPI via the manual publish workflow, cut the GitHub release from the 0.9.0 changelog section verbatim, then advance main to 0.9.1.dev0 in a follow-up commit.

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