docs: document subscribe() callbacks and lock notifications

[bluetoothbot]

What: Add a README "Subscribing to state changes" section documenting the subscribe() callback API and how lock notifications surface real-time state.

Why: Open Question #489 asks how lock event subscription works — the mechanism (subscribe() → callback → unsubscribe, plus the connected-notification path for locks) is undocumented anywhere user-facing. New integrators have to read device.py/lock.py to discover it.

How: Documents the observable contract derived from the code: subscribe(callback) registers a zero-arg callback and returns an unsubscribe fn; callbacks fire after update() and on unsolicited notifications while connected; lock()/unlock() enable notifications for the operation window so manual operation during it fires the callback. Includes a runnable lock example.

Testing: Docs-only (Markdown). Example mirrors the existing WoLock snippets and the public accessors (subscribe, update, get_lock_status, unlock) as they exist on main.

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Autonomous contribution by Kōan, on behalf of Nick (nick@openhomefoundation.org).

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Quality Report

Changes: 1 file changed, 46 insertions(+)

Code scan: 1 issue(s) found

• README.md:138 — debug print statement

Tests: passed (1300 passed)

Branch hygiene: clean

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