fix: pass slash trigger context to provider catalogs

[SkyNotSilent]

Summary

• Pass slash dropdown trigger context through the shared provider command catalog interface.
• Allow provider catalogs to receive both includeBuiltIns and triggerChar so providers can distinguish / from $ when needed.
• Allow Codex provider slash built-ins such as /compact to appear when the composer is triggered with / at the start of the input.
• Keep $ scoped to Codex skills so $skill completion does not mix in slash-only provider commands.

Why

Provider-native command discovery should be decided by the provider catalog, while the shared dropdown should supply the trigger context. Previously the dropdown wrapper always requested provider entries with includeBuiltIns: false, so Codex-native slash commands such as /compact were not discoverable even though sending /compact manually worked.

This keeps Claudian-local commands such as /clear handled locally, while provider-native commands and skills are exposed through the correct trigger namespace. Providers that only use / can ignore triggerChar; providers with multiple trigger namespaces can now make that decision at the catalog boundary.

Related context checked

• Related but not duplicate: Built-in Claude Code commands missing from slash command autocomplete #438 covers Claude built-in command autocomplete.
• Related but not duplicate: 关于 claudian 会话面板中 skills 没有快速提示的问题 #88 covers older skill quick-prompt UX.
• Related history: feat: enable /compact command with UI boundary indicator #207 enabled Claude /compact discovery.
• Possible overlap: fix: rank exact slash command matches first #757 also touches SlashCommandDropdown for exact-match sorting, but this PR changes provider-entry context propagation rather than result ranking.

Validation

• npm run test -- --selectProjects unit --runTestsByPath tests/unit/shared/components/SlashCommandDropdown.provider.test.ts tests/unit/features/chat/tabs/TabManager.test.ts tests/unit/providers/codex/commands/CodexSkillCatalog.test.ts tests/unit/providers/claude/commands/ClaudeCommandCatalog.test.ts tests/unit/providers/pi/commands/PiCommandCatalog.test.ts tests/unit/providers/opencode/OpencodeCommandCatalog.test.ts --runInBand
• npm run typecheck
• npm run lint
• npm run test -- --runInBand
• npm run build
• git diff --check