Fix flaky skill watch test: add non-recursive watchers on skill subdirectories

[Copilot]

In scan mode, the watch skill file changes integration test flakily times out because content-only modifications to SKILL.md (two levels deep: skillsDir/skill-name/SKILL.md) are sometimes not detected on macOS CI VMs.

Root cause

Skills were watched exclusively via a recursive @parcel/watcher on skillsDir. On macOS VMs, this recursive watcher can fail to fire for content-only changes deep in the tree. Agents don't have this problem because they use a non-recursive fs.watch/kqueue watcher on a directory whose direct children are the watched files.

Fix

In _scanRoot for DiscoveredType.Skill, add an explicit non-recursive watcher on each discovered skill subdirectory (trigger = the SKILL.md file URI):

// before: only recursive watcher on skillsDir
addWatch(watchRootUris, rootUri, root.recursive ?? false, rootUri);

// after: also add non-recursive watcher on each skill's subdirectory
addWatch(watchRootUris, child.resource, false, skillFile);

When SKILL.md is modified, e.affects(skillFile) returns true immediately — a more specific, reliable trigger that doesn't depend on @parcel/watcher's recursive event delivery. This mirrors how discover mode watches individual skill file paths.

Tests

• Updated installs watchers for roots that contain discovered customizations to assert skill subdirectories (e.g. .github/skills/bar) are now watched non-recursively in addition to the recursive skillsDir watcher.
• Added fires onDidChange when an existing SKILL.md file is modified to directly validate content-change detection via the new non-recursive watcher.

• Fixes Flaky AHP integration test #327447

[rzhao271]

Closing in favour of Rob's PR from earlier today.