refactor(settings): move refresh_open_files toggle to NBI config

[pjdoland]

Summary

Issue #337 asks that the refresh_open_files_on_disk_change setting (introduced in #330) live in NBI's own config and Settings dialog rather than JupyterLab's Settings Editor, for consistency with the other user-toggleable booleans (enable_explain_error, enable_output_followup, enable_output_toolbar). This PR relocates it.

Solution

The setting now lives in ~/.jupyter/nbi/config.json (read via NBIConfig.refresh_open_files_on_disk_change) and renders as a checkbox in a new External changes section of the NBI Settings dialog. It is wired through the standard admin-policy triad: a FEATURE_POLICY_SPEC entry, a TraitletEnum, and a NBI_REFRESH_OPEN_FILES_ON_DISK_CHANGE_POLICY env var, so deployments can pin it force-on/force-off without touching code.

Key changes:

• notebook_intelligence/extension.py: FEATURE_POLICY_SPEC entry, new refresh_open_files_on_disk_change_policy TraitletEnum, _build_feature_policies_response user value, ConfigHandler.valid_keys + locked_keys gating.
• notebook_intelligence/config.py: refresh_open_files_on_disk_change property (default True).
• src/api.ts: extends FeaturePolicyName union, the featurePolicies getter, and the defaultOpen set so the watcher's boot-window read survives until capabilities land.
• src/index.ts: the watcher's isEnabled callback now re-reads NBI capabilities every tick, so toggling in the Settings dialog applies without a Lab restart.
• src/components/settings-panel.tsx: new "External changes" section with a CheckBoxItem that honors featurePolicies.refresh_open_files_on_disk_change.locked.
• schema/plugin.json: removed the orphan property (the schema retains jupyter.lab.toolbars and jupyter.lab.shortcuts blocks).
• Docs: README "Admin policies" table row and a new docs/admin-guide.md section.

Note that the prior home for this setting was only exposed in 5.0.0-alpha, so a runtime migration of stale JL Settings Registry values is not included; an alpha user who toggled it false will revert to the default true and can opt out again in the new dialog.

Testing

• pytest tests/ --ignore=tests/test_claude_client.py -q: 1066 passed.
• jlpm tsc --noEmit: clean.
• jlpm lint:check: clean.
• jlpm jest: 281 passed.
• Three new pytest cases in tests/test_cell_output_features_response.py pin the capabilities-response shape across user-choice, force-on, and force-off.
• Playwright verification in a local Lab session: confirmed the dialog renders the new section, the toggle round-trips to ~/.jupyter/nbi/config.json and the capabilities response, and NBI_REFRESH_OPEN_FILES_ON_DISK_CHANGE_POLICY=force-off correctly disables the checkbox and returns {enabled: false, locked: true}.

Two reviewer rounds (six agents each: three /simplify and three persona reviewers covering JL extension lifecycle, frontend a11y, and API contract). All fix-before-merge findings were applied in round 2; remaining notes are scoped as follow-ups.

Risks / follow-ups

• The featurePolicies getter in src/api.ts allocates per call; the watcher tick reads it every 3 s. Round 2 efficiency review confirmed this is sub-microsecond and dwarfed by the watcher's own filesystem fan-out. A future pass could memoize the getter against this.capabilities identity for broader benefit, but it isn't load-bearing for this change.
• model-config-section-header is a styled <div> rather than a semantic heading; this is pre-existing across the whole Settings dialog and out of scope here.

Screenshots

Default (unlocked, checked) and force-off (locked, disabled) attached locally at nbi-settings-external-changes.png and nbi-settings-force-off-locked.png; will drop into this description via the web UI.

Closes #337.