feat(skills): add inline skill references and picker support - #1298

Summary

Add inline skill reference support across the runtime prompt and FlowChat input UX.

Fixes #951
Fixes #1285

Type and Areas

Type:

• feature
• UI/UX

Areas:

• Rust core
• web UI
• i18n

Motivation / Impact

This introduces a unified [$skill] reference model for runtime skill prompts and chat input interactions.

Users can now:

• trigger skills directly from slash or $ inline pickers
• see selected skills rendered as pills consistently
• insert skills from the legacy boost menu without spacing or line-break glitches
• navigate command picker items cyclically with the arrow keys

On the runtime side, skill snapshot and skill listing output now render as inline <skill name="...">...</skill> entries and the skill guidance explicitly documents the [$SkillName] invocation format.

Verification

• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run src/flow_chat/components/RichTextInput.test.tsx src/flow_chat/utils/skillPromptReference.test.ts

Reviewer Notes

• This staged set spans both runtime prompt formatting and FlowChat input UX changes because they are part of the same skill-reference feature.
• Locale strings were updated for the new command picker section label.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.