feat: add TUI autocomplete for Inline Skill References

[bkyssn]

Parent

• feat: support inline skill references with $ autocomplete #282

What to build

Add TUI autocomplete for Inline Skill References so a user can discover and insert $skill-name while composing a message. Accepting a candidate must keep the editor open, and submitting the completed message must use the Session expansion delivered by #316.

The autocomplete provider must compose with Pi's current provider. Native /skill:name completion and @path/to/file completion remain unchanged.

Acceptance criteria

• Typing $ at the start of input or after horizontal whitespace opens Inline Skill completion in the TUI.
• Candidates contain only Skills currently loaded in the Session.
• Each candidate preserves the Skill description supplied by Pi.
• Accepting a candidate inserts the literal $skill-name at the active reference.
• Tab accepts the selected candidate without submitting the message.
• Completion reads Pi's current Skill command projection dynamically and does not maintain a separate cache.
• Reloaded Skill state is reflected in later completion requests.
• Inputs that are not Inline Skill References delegate to Pi's existing autocomplete provider.
• Native /skill:name completion remains unchanged.
• Native @path/to/file completion and file-reference behavior remain unchanged.
• Provider-level tests cover candidate filtering, insertion, dynamic Skill state, and delegation.
• A TUI Editor test proves Tab acceptance updates the editor without invoking submit.
• Manual validation follows the runtime provenance procedure and proves exactly one OpenPI source points at the tested checkout.
• The repository's complete check and test commands pass from the current main branch.

Blocked by

• test: preserve native Pi Skill invocation and lifecycle #316

[YinFengWindy]

I would like to take this issue. I will implement the TUI autocomplete for inline Skill references while preserving the existing /skill:name and @path/to/file completion behavior. Implementation approach: wrap Pi current AutocompleteProvider at the TUI editor seam; when the active reference begins at input start or after horizontal whitespace, derive the $ prefix and build candidates dynamically from the Skills currently supplied by the Session, preserving Pi-provided descriptions and inserting the literal $skill-name through the editor insertion path without submitting. All other inputs will delegate unchanged to Pi, including native slash and file-reference completion. I will add focused provider tests for boundary filtering, descriptions, delegation, dynamic Skill reload, and a TUI editor test proving Tab accepts without submit. The repository check, test suite, and runtime provenance validation will be run before submission. Note that this issue is currently blocked by #316.

[bkyssn]

感谢认领。这个子 Issue 的实现已经由 #321 覆盖，相关代码、测试和 CI 均已完成。为避免重复开发，请暂缓继续实现；#321 合并后本 Issue 会自动关闭。

[tt-a1i]

@YinFengWindy 感谢认领。维护者正在统一整合 #321 与 #322：以 #322 为主，纳入 #321 已完成的补全与测试，整合和修复由我们负责。请先暂停重复实现；如果你已经有不同的代码或测试成果，请贴分支或 PR，我们会在收口前查看，避免遗漏贡献。进展统一记录在 #322。