perf(watch): prune excluded subtrees during setup

Problem:
The LSP client excludes paths such as Git objects and installed
dependencies from file watching. However, watchdirs() still traverses
these subtrees before filtering which directories receive watchers,
paying the filesystem and Lua traversal cost for excluded content.

Solution:
Prune excluded subdirectories during the initial walk, and document
that excluding a directory also excludes its descendants.

Using the LSP client's existing **/node_modules/*/** exclusion on a
synthetic macOS tree with 100 installed packages and 4,000 files:
                     Before  After
  Directory scans       503    103
  Entries inspected    4502    202
  Directory watchers    103    103

The measurements use real filesystem calls and watcher handles, with
identical watcher paths for this exclusion.

Ref: #23291

AI-assisted

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