[FEATURE] @-mention file injection: attach vault notes as context by typing @filename

[ScottKirvan]

Summary

Allow users to type @filename (or @[[wikilink]]) in the chat input to attach a specific vault note's content as inline context for that message.

Motivation

This is a standard expected feature across competing plugins:

• Claudian (YishenTu): @-file mentions with tag-based exclusion
• Agent Client: @notename vault-aware context with auto-mention mode
• Obsidian AI CLI: @filename.md references
• Roasbeef/obsidian-claude-code: @[[filename]] with autocomplete

Currently Cortex relies on Claude reading files itself via its tools. @-mention gives users explicit, predictable control over what context is included.

Proposed behavior

• Typing @ in the chat input triggers an autocomplete dropdown of vault note names
• Selecting a note injects its content (or a reference) into the message before sending
• Related to [FEATURE] Inline selection context: send highlighted text to Cortex as context #17 (inline selection context) — together these cover the full 'explicit context control' use case

Notes

Should be compatible with the stdin-based prompt delivery architecture — the @-mentioned content is resolved client-side before writing to stdin.

[ScottKirvan]

Shipped. Type @ in the chat input to open an autocomplete dropdown. The currently open note is pre-selected — press Enter to attach it immediately. Non-markdown files (e.g. .fountain, .txt) show their extension in the dropdown. Configurable file extensions via Settings → @-mention file types (default: md, fountain, txt).