perf(skills): reuse walk inventory for host loading

[jif-oai]

Why

Thread startup loads repository skills through the primary remote environment. This still used the older host skill loader, which recursively issued directory and metadata requests over the exec-server connection. On a high-latency connection, one skill scan could turn into hundreds of round trips.

Executor-selected skill roots already avoid this pattern: they use one bounded filesystem walk, reuse the returned inventory, and overlap independent reads.

What changed

• Extract the URI-native walk inventory into a shared private discovery module.
• Use that discovery path for both environment-owned and host/repository skill roots.
• Reuse the walk inventory to avoid per-skill metadata existence checks when the inventory is complete.
• Canonicalize host skill identities and parse skill files with bounded concurrency.
• Prune hidden host directories during traversal, preserving visible aliases and walk limits.
• Cache namespace probes by ancestor and keep all remote namespace reads under the same concurrency bound.
• Keep safe metadata probes for incomplete walks, file symlinks, and case aliases.

Dependency

#31570 is now merged. It provides the optional pruneHiddenDirectories walk flag used here. That flag defaults to false and is omitted when disabled.

No other exec-server API changes are introduced.