fix: make the UIA focus watchdog opt-in

[Jeomon]

Refs #332. Behaviour change: the UIA focus watchdog no longer starts by default.

Why

While working #332 I went looking for what the watchdog actually feeds, and the answer is: nothing. Its only consumer in production is Tree.on_focus_change, and that method in full debounces duplicate events and writes one DEBUG log line. _last_focus_event is read nowhere except its own debounce check, and set_structure_callback / set_property_callback are called from nowhere at all.

Git history explains it. There used to be a _on_structure_change doing the real work — mutating tree_state.interactive_nodes, adding nodes on ChildAdded, removing them on ChildRemoved, rebuilding subtrees on ChildrenInvalidated, updating them on ChildrenReordered. That was deleted, and the log-only _on_property_change went with it. What survives is the delivery mechanism with its payload removed.

So the current cost/benefit is a dedicated STA thread, a long-lived UIA event subscription, COM construction at import time (the hazard #398 had to guard), and exposure to the native access violation in PumpEvents that kills the entire server with no Python traceback — in exchange for a debug log line.

What changed

WINDOWS_MCP_WATCHDOG becomes opt-in: on, 1, true, yes or enabled starts it, unset or anything else leaves it off. manifest.json's user_config default flips to false, and its description no longer claims the watchdog "keeps the accessibility tree current" — untrue since the structure-change code was removed. README and CLAUDE.md updated to match.

Anyone relying on the watchdog can still turn it on, and #399's backoff fix means it behaves better if they do.

Verification

The accessibility tree is built on demand for every tool call, so nothing about tool behaviour depends on this. Confirmed against the real stdio server rather than assumed — initialize + tools/list under both settings:

                   env |  _watchdog_enabled() | threads in server
                 unset |                False | 32
                    on |                 True | 36
                   off |                False | -
                  true |                 True | -
                 false |                False | -

All 20 tools served either way, clean exit both times; the default genuinely does not start the thread, and the manifest's boolean maps through correctly in both directions.

Full suite: 604 passed.

On #332

This does not "fix" the crash so much as stop shipping the thing that causes it to every user by default, which for a component with no current function is the honest trade. If the tree-cache maintenance is ever restored, the watchdog can be turned back on — ideally on a polling loop rather than native callbacks, so an access violation in UIA cannot take the server down. I'd suggest keeping #332 open until then.