## Why
CCA 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.
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