feat(skills): skills.create_dir + shared-pool auto-commit

[Frosti7]

Summary

Adds two complementary capabilities to the skills system:

1. skills.create_dir config option — when set in config.yaml, newly created skills are written to that directory (e.g. a shared fleet repo like ~/.hermes/shared-skills) instead of the profile-local ~/.hermes/skills/. Falls back to the local dir when unset (current behavior unchanged).

2. Shared-pool auto-commit — when a skill write (create or patch) lands in a git-tracked external/shared skills directory, the change is committed automatically (skills: <name> update (agent write)), keeping the shared repo clean with full history. Failures are silent — the write itself already succeeded.

Motivation

For fleets of multiple Hermes profiles (agents), skills are typically shared via skills.external_dirs pointing at a common git repo. Today:

• New skills land profile-local and must be manually promoted into the shared repo.
• In-place edits to external skills are supported but leave the repo dirty (uncommitted), so history is lost and the next git reset/checkout can wipe the change.

This makes "shared by default, improve together" work natively: agents create and edit skills directly in the shared repo, with git history as the audit trail.

Notes

• _resolve_skill_dir honors skills.create_dir (expanduser/expandvars, absolute or relative to HERMES_HOME).
• The auto-commit only fires for external dirs that are git repos; it never blocks or modifies the write itself.
• Default behavior (no create_dir configured) is identical to today.

Related: #5535 (skills.shared shorthand — different mechanism, same goal).

[Frosti7]

Closing: this PR accidentally included an unrelated local commit (context-compression governor). Reopened as a clean PR containing only the skills.create_dir change.