Edits targeting the agent skills directory (project `.agents/skills/`
and global `~/.agents/skills/`) always prompt regardless of permission
settings, because they route through `authorize_always_prompt` which
never consults the persisted `always_allow` rules.

The permission prompt still rendered an "Always allow" button, though.
Clicking it wrote an `always_allow` rule into settings that was then
silently ignored on the next skills edit, so users were re-prompted
every time despite having chosen "always allow" — exactly the confusion
reported in AI-346.

This keeps the always-prompt behavior intact and just hides the
misleading "Always allow" / "Always deny" options for skills, leaving
once-only Allow/Deny. Applies to all mutating skills tools: `edit_file`,
`write_file`, `copy_path`, `move_path`, `delete_path`, and
`create_directory`.

Implementation:
- New `ToolPermissionScope::AgentSkills`, set via
`ToolPermissionContext::for_agent_skills()`.
- `build_permission_options` returns flat once-only Allow/Deny for that
scope (mirrors the existing `SymlinkTarget` handling).
- Wired into both authorization entry points (`authorize_file_edit` and
`authorize_with_sensitive_settings`).
- Tests assert no `AllowAlways` option is offered for each affected
tool.

Settings-file prompts (`.zed/settings.json`, global config) are
intentionally left unchanged.

Closes AI-346

Release Notes:

- Removed the misleading "Always allow" button from agent permission
prompts for edits to the agent skills directory, since those edits
always require confirmation.