docs: document the contract, the client, the security model, and 0.3.0's breaks

[V3RON]

Stacked on #98. PR 5 of 5 implementing ADR 0003 (§11, §12, Consequences).

New and rewritten documentation

• docs/CLIENT.md (new) — the programmatic client: connecting, the four abort behaviours, and the contract that it never reconnects and never retries, so reconnect policy is the caller's. Linked from README.md and docs/ABOUT.md.
• docs/ARCHITECTURE.md — a new section on the contract module, the single dispatcher serving every transport, roles, and the principal/requester/owner distinction. Makes explicit that HTTP calls the dispatcher in-process rather than routing through the loopback socket.
• The security model, stated plainly, as §4 and §5 require: socket identity is cooperative — every socket peer is the same OS user, which is the real trust boundary, and ownership checks protect against accidents, not a hostile local process. Documents both admin credentials and the CLI's resolution order, and that doctor.run without fix is agent-visible and read-only but shells out per device.

Breaking changes documented for 0.3.0

• MCP: timeout_seconds → timeoutMs. A unit change as well as a rename — an un-updated caller is silently wrong by 1000×. Documented prominently; it is the highest-risk item in this release. Also slim → device.featureProfile.
• CLI: --json and stderr lines are contract values; the snake_case keys are gone. Exit codes unchanged.
• HTTP: bodies are contract types; GET /v1/leases/:id returns 404 rather than 403 for another requester's lease.
• Wire: protocol 3, no compatibility shim. PROTOCOL_VERSION_UNSUPPORTED on mismatch, the client never restarts the daemon, and daemon.stop is accepted at any protocol version the daemon has spoken — while still requiring the admin role.

Bug fixes documented as such

The allowDownload clamp now applying to HTTP, and an HTTP request during startup parking rather than being refused. Both were divergences the ADR set out to close.

Docs that disagreed with the code

Found and fixed, rather than papered over: HTTP's "same role interfaces" description and its startup-refuse behaviour; the 403/404 change; undocumented allowDownload clamping; MCP's breaking changes documented nowhere; README.md's pre-ADR snake_case example; stale slim references in known-pitfalls.md; and ARCHITECTURE.md bullets describing code paths this stack deleted.

Follow-ups recorded in known-pitfalls.md

True cancellation during provisioning is not implemented — aborting mid-provision releases the grant rather than stopping the work, exactly as §10 requires be recorded. The HTTP tracker and notice buffer are named as the last frontend-held state, which #72 removes.

docs/EVENTS.md was already in sync — PR 2 documented ownerId on lease.released/lease.expired in the same change, per events rule 8.

Smoke tests (§12)

All four exist. Client, CLI and MCP are explicitly labelled as the §12 smoke test. HTTP's round-trip lifecycle test covers it in substance but is not labelled and runs against a fake dispatcher — noted rather than reworked.

Notes for reviewers

• CHANGELOG.md is new and hand-written. The repo generates it via release-it/conventional-changelog, but this stack's commits carry no ! or BREAKING CHANGE: footers, so automatic detection would not have surfaced any of the breaks above. Worth deciding whether future commits adopt that convention.
• src/contract/*.ts still carry stale in-code comments from earlier PRs ("PR 2 adds…", "no handler exists yet" for token operations) that no longer match reality. Not documentation, so untouched here; flagged for a cleanup pass.
• ADR 0003's status is now Accepted, with the index synced.