feat(context): share selected IDE context with a thread

[coygeek]

Summary

Allow users to attach the active editor, selection, and open-file context to a T3 Code turn through an explicit toggle.

Problem to solve

T3 Code users working from an IDE must manually paste file and selection context even when the editor already knows the relevant location, causing stale or incomplete prompts.

Proposed behavior

Add an IDE context connection and a visible on/off control in the composer. When enabled, the next turn receives the selected file, selection range, and optionally open-file list with clear provenance; users can inspect and remove the attachment before sending.

Acceptance criteria

• The composer indicates whether IDE context is connected and enabled.
• A user can preview, remove, or disable context before sending.
• Context includes repo-relative paths and selection ranges, is bounded in size, and is not attached when disabled.
• Reconnects and remote clients preserve the setting without leaking machine-local absolute paths.
• Unsupported clients explain the unavailable integration instead of silently claiming it is active.

Affected area

IDE bridge and authentication, attachment contracts, composer state, server turn context, and web/desktop/mobile capability reporting.

Non-goals

This does not add an editor, language server, or automatic indexing of an entire repository.

Alternatives considered

Pasting context manually remains useful for unsupported editors but cannot provide a reliable toggle or provenance.

Supporting context

Official OpenAI slash-command documentation lists /ide-context to turn shared IDE context on or off: https://learn.chatgpt.com/docs/reference/slash-commands#available-slash-commands. The inspected T3 Code client has composer attachment and mention logic but no IDE-context command or shared IDE bridge contract.

Related upstream discussion: IDE / Code-Editor integration #7886. This proposal is limited to explicit editor-context attachment, not a full IDE.

Analysis basis: source inspection of T3 Code revision 9d4bb550a658. Statements about missing UI or contracts refer to that revision, not every provider-native capability or unmerged branch. This is a feature proposal based on source review, not a runtime bug reproduction.