docs: add root architecture reference

[owainlewis]

Summary

Replace the older docs-site architecture page with one root ARCHITECTURE.md modeled on Block Buzz’s numbered architecture reference. The new source covers runtime boundaries, process and message lifecycles, channels, agent adapters, durable data, jobs, concurrency, security, recovery, source ownership, and verification. Repository and docs-site navigation now point to that single source of truth.

Why

The architecture reference should be visible from the repository root, comprehensive enough to guide changes, and maintained in one place. Root documentation links are now included in the existing documentation link test.

Test plan

• cargo fmt --all --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked (367 tests passed)
• bash tests/install.sh
• bash tests/release-version.sh
• mkdocs build --strict in a temporary environment from requirements-docs.txt
• Verified exactly one ARCHITECTURE.md remains
• Independent agent review completed with no unresolved findings

Risks

Low. This is documentation and documentation-test coverage only. The main risk is architecture text drifting from implementation; source links and the expanded link test reduce navigation drift, while factual claims were checked against the current code.

Related issue

None.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Correct the state rollback guarantee

When saving a newly created, rotated, or started backend session fails, Store::session_for, Store::rotate, and Store::mark_started have already mutated self.state and do not restore its previous value; only set_cursor implements rollback. A subsequent message in the same process can therefore observe session state that was never persisted, contrary to this architecture guarantee. Narrow the statement to cursor writes or add rollback to the session mutations.

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