feat: line-range @mention via Option+K and Shift+drop

[wanzirong]

Summary

• Option+K shortcut: When an editor has an active selection, pressing Option+K (Alt+K on Windows/Linux) inserts @filename#startLine-endLine into the chat input. At send time, the referenced lines are read from the vault and injected as <editor_selection path="..." lines="..."> XML into the prompt.
• Reading mode support: When no line numbers are available (reading/preview mode), Option+K instead inlines the selected text directly as an <editor_selection> block.
• Shift+drop: Holding Shift while dragging a file from the Obsidian file list onto the Claudian panel inserts @filepath into the input (without opening the file).

Technical Notes

• Uses document capture-phase listeners for Shift+drop to intercept before Obsidian's own drop handler.
• e.code === 'KeyK' (not e.key) for Option+K to work correctly on macOS where Option+K produces the composed character ˚.
• Send-time resolution happens in InputController.sendMessage (async) before runtime.prepareTurn, keeping prepareTurn synchronous.
• Manually typed @file#line is never resolved — only shortcut-inserted mentions are whitelisted via FileContextState.attachLineRangeMention.

Test Plan

• In source mode, select lines in editor → press Option+K → verify @filename#start-end inserted in input
• Send the message → verify <editor_selection> XML with correct line content appears in prompt
• In reading mode, select text → press Option+K → verify <editor_selection> block inlined directly
• Press Option+K with no selection (cursor only) → verify nothing inserted
• Shift+drag a file onto Claudian panel → verify @filepath inserted
• Normal drag (no Shift) → verify file opens as usual, no mention inserted
• Manually type @file#1-5 → send → verify no XML injection

🤖 Generated with Claude Code

[YishenTu]

current selection indicator already supports selected content as well as line number, and it's all automatic.

what's new in this pr?

[wanzirong]

Thanks for the context! Here's what's new:

Inspiration: This is modeled after the Claude Code VS Code extension's line-range mention feature (https://code.claude.com/docs/en/vs-code).

What's different from the existing selection indicator:

The existing selection indicator automatically captures the current active editor selection — which is great, but it's limited to a single selection at a time and is replaced whenever the selection changes.

This PR adds:

1. Option+K (Mac) / Alt+K (Windows/Linux) — Press the shortcut while text is selected to pin that selection as an @filename#startLine-endLine mention in the input. You can do this multiple times to reference multiple independent line ranges from the same or different files in a single message.
2. Shift+drag a file onto the Claudian panel to insert @filepath as a mention token without opening the file.
3. At send time, each pinned mention is resolved into <editor_selection path="..." lines="...">…content…</editor_selection> XML so the model sees the exact lines.

The key difference: the existing indicator is ephemeral and single-selection. This lets you deliberately pin multiple ranges before hitting send.

[YishenTu]

i like the idea about the multi selection insertion, but i think the file-drag feature is not aligned with current design, the chip is to "link" a session to a note, not to display all the files that has been mentioned.

if i were to support multi-line selection, i will simply create a new hotkey to insert the selected content path and line number, plain and simple.