docs(skills): clarify external dir mutations

[helix4u]

What does this PR do?

Updates the External Skill Directories docs to match current skill_manage behavior on main.

The docs previously said external dirs were read-only and that agent-created edits always wrote to ~/.hermes/skills/. Current code and tests allow skill_manage to patch, edit, write supporting files, remove supporting files, and delete existing skills where they are discovered, including under skills.external_dirs.

This PR changes the docs to say new skills are still created locally, while existing external skills can be updated in place when the external directory is writable. It also updates the example directory comment so it no longer labels external dirs as read-only.

Related Issue

Related: #25083

Also checked related PRs before opening this docs-only update:

• fix(skills): let skill_manage modify skills in external_dirs in place #17512 merged the current in-place mutation behavior.
• fix(skills): copy-on-write for agent edits of external skills #5407 proposed copy-on-write behavior but is closed.
• fix(skills): protect pinned external skills from mutation #27993 protects pinned external skills from mutation but preserves unpinned external mutation compatibility.
• feat(skills): skills.shared shorthand for cross-profile sharing #5535 is adjacent shared-skills work, not a duplicate docs update.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Updated website/docs/user-guide/features/skills.md to clarify that skill_manage creates new skills under ~/.hermes/skills/, but modifies existing skills where they are found, including writable external_dirs.
• Replaced the read-only external-dir example label with wording that matches current behavior.

How to Test

1. Run git diff --check HEAD~1 HEAD.
2. Confirm the command prints no whitespace errors.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: Ubuntu / WSL, docs-only markdown change

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

Docs-only change. Validation:

$ git diff --check HEAD~1 HEAD
# no output

[kshitijk4poor]

Salvaged and merged via #29444 (commit 4d2df86). Your commit was cherry-picked onto current main with authorship preserved — thanks for catching the docs/code mismatch from #25083. The immutable: true feature request itself stays open at #25083.