Users can open skill folders in Terminal

[ryota-murakami]

Problem / Motivation

When users want to run shell commands inside the universal source folder (~/.agents/skills/) or any agent-specific skills folder (e.g. ~/.claude/skills/) — checking symlink targets with ls -la, running git inside a skill, or simply exploring — they currently have to open a terminal manually and re-type or remember the path. This breaks flow for any hands-on inspection or fix.

Acceptance Criteria

• User can open the universal source skills folder in a terminal from within the app
• User can open any agent-specific skills folder in a terminal from within the app
• The action is reachable from the surfaces where users already see these folders (sidebar agent list, Universal entry, etc.) without typing a path
• The terminal opens with its working directory already set to the chosen folder, so the user does not have to cd after launch
• If the target folder does not exist, the user sees a clear, non-technical message rather than a silent failure or a raw error

Examples

• A user wants to run ls -la to inspect symlink targets in an agent's skills folder. One click opens a terminal already in that directory.
• A user spots a broken skill and wants to manually git pull inside the source directory. The app drops them into a terminal there, no typing required.
• A user is debugging permissions on a single agent's skills folder. The terminal opens preset to that agent's path.

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Technical design & implementation plan will be produced at pickup time via /plan-eng-review.

Related: #103 (open in Finder) — sibling feature; likely shares discoverability surface and path-validation logic.