TUI: Default to unified mentions, deprecate gate, polish mentions rendering

[canvrno-oai]

This change makes the unified @mentions the default TUI behavior, removes the old split between file-only @ search and tool mention behavior, folds the legacy file-search popup into the unified mentions flow, and keeps the old mentions_v2 config flag as a compatibility no-op so existing configs still load without warnings.

It also adds a few small improvements to the mentions feature around mention rendering and history round-tripping for plugin/tool mentions in message edit scenarios. Plugin selections now insert @ mentions with better casing, and saved history preserves the visible sigil so recalled messages look the same as what the user typed.

• Ungates unified @mentions and downgrades mentions_v2 to a removed compatibility flag.
• Deletes the standalone legacy file-search popup and keeps file search inside the unified mentions popup.
• Preserves @ sigils when encoding/decoding mention history for tool/plugin paths.
• Improves plugin mention insertion so display names/casing are reflected more cleanly in the composer.
• Update composer to render user-entered plugin mentions in the same color as the mentions menu. ALso applies to recalled/edited messages.
• Left/right arrows no longer switch unified-mention search modes after an @mention has already been accepted (Ex: arrowing left through a composed message that contains @mentions).

Steps to test

• Clear/remove any flags for mentions_v2 enablement from config.
• Type @ in the TUI composer and verify filesystem/plugin/skill results are displayed in the unified mentions menu.
• Select a plugin mention from the @ popup and confirm the inserted text is an @... mention with casing, then recall/edit the message and confirm it still renders as @....
• Mention a skill and verify that skills still insert as $skill mentions rather than @ mentions.

[canvrno-oai]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: c8dd875041

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".