ENG-1615: Only add backlink when text is selected in "Create discourse node" command (Obsidian)

[trangdoan982]

https://www.loom.com/share/a3267f6e354d4b3e9c61ab32758d7bd9

Summary

• createModifyNodeModalSubmitHandler now takes editor as optional — only passed when an active editor is available
• Backlink (editor.replaceSelection) for linking to an existing node is now gated on editor.somethingSelected()
• create-discourse-node command switched from editorCallback to callback, resolving the case where the command is invoked from outside an active editor (editor becomes undefined)
• New node creation path already conditionally used editor via createDiscourseNode — no change needed there

Fixes: https://linear.app/discourse-graphs/issue/ENG-1615

Test plan

• With no text selected, run "Create discourse node" → node is created, no backlink inserted at cursor
• With no text selected, run "Create discourse node" and link to an existing node → node linked, no backlink inserted
• With text selected, run "Create discourse node" → node is created and backlink replaces the selected text
• Run "Create discourse node" from outside any editor (e.g. file explorer focused) → modal opens, node is created, no crash

🤖 Generated with Claude Code

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[linear-code]

ENG-1615 Remove behavior of adding backlink to page from node creation via "Create discourse node" command (Obsidian)

[supabase]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[maparent]

This looks good in principle, but I thought of an edge case:
What if the existing selection already has [[square brackets]], because the user added them preemptively?
I think we should test for that. Feel free to push back and decide it's scope creep, but I'd like it on record as a decision either way.

ETA: I tested, and this gets rejected as having invalid characters. Good enough.

[maparent]

Approved on second thought.