fix(coding-agent): hide plugin skills from slash commands

[roboomp]

Repro

A Claude marketplace plugin with skills/understand/SKILL.md was loaded as both a skill and a bare slash command. Reproduced with bun test packages/coding-agent/test/discovery/claude-plugins.test.ts --test-name-pattern "exposes plugin skills as bare slash commands", which passed on the old code because /understand expanded to the skill body.

Cause

packages/coding-agent/src/discovery/claude-plugins.ts called loadSkillSlashCommands() from loadSlashCommands(), scanning each plugin skills/ directory and appending those skills to the slash-command capability in addition to the normal skill provider.

Fix

• Removed plugin skill-to-slash-command mirroring from Claude plugin slash-command discovery.
• Kept plugin commands / slash-commands directory loading unchanged.
• Updated packages/coding-agent/test/discovery/claude-plugins.test.ts to assert plugin skills remain loadable as skills but absent from slash commands, while command manifest entries still load.
• Added the coding-agent changelog entry for fix(coding-agent): Claude plugin skills should not appear as slash commands #2645.

Verification

bun test packages/coding-agent/test/discovery/claude-plugins.test.ts --test-name-pattern "keeps plugin skills out of slash commands|reads slash commands directory from plugin manifest commands field|reads slash commands directory from plugin manifest slash-commands field" passed. bun test packages/coding-agent/test/discovery/claude-plugins.test.ts passed. Fixes #2645