feat(tui): show active skills in session sidebar - #48461

Issue for this PR

Closes #48355

Type of change

• New feature

What does this PR do?

Adds an Active Skills section to the TUI session sidebar. It shows only skills that were actually loaded through the skill tool in the current session, rather than every discovered skill. The panel includes each skill's source (built-in, project, global, or external) and stays empty until a skill is loaded.

The harness now also adds a compact <loaded_skills> reminder to the system prompt on every turn. It tells the model which skills are already present in message history and not to load them again. Compaction carries the same skill list through the summary and auto-continue prompt, so the reminder survives a long session.

The skill list is permission-filtered for the session agent. If an old session refers to an agent that no longer exists, no skills are injected rather than falling back to the unfiltered list.

How did you verify your code works?

• Ran bun typecheck in packages/plugin, packages/tui, and packages/opencode.
• Ran bun dev from packages/opencode locally.
• Loaded the clickcast skill in a session: the sidebar showed only clickcast under Active Skills.
• Asked the model which skills were loaded on the next turn: it answered clickcast without invoking the tool again.

Screenshots / recordings

Tested in the terminal TUI locally. The sidebar showed:

Active Skills
• clickcast external

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR