Add TreeHacks 2026 (closed; Feb 13–15, 2026)

[ChimdumebiNebolisa]

Summary

• Add Stanford TreeHacks 2026 as closed (event dates Feb 13–15, 2026 have passed; no announced next cycle)
• Add Stanford, CA geocode; listing goes to ARCHIVE (not live README Register)
• Do not mark registration open or invent a future cycle

Closes #148

Evidence

• Official site https://treehacks.com/ still presents TreeHacks 2026 / FEB 13–15, 2026
• /register and /apply 404; no live participant application flow for a future cycle

Test plan

• Listing state=closed, active=false
• ARCHIVE regenerated; no Register badge for TreeHacks

[Jose-Gael-Cruz-Lopez]

Synced this branch with main and corrected the listing state to closed.

treehacks.com still serves the Feb 13–15, 2026 event — that date is ~5 months past. The page contains no "2027" string anywhere, and no apply/register link at all (only about/tracks/speakers/sponsors/FAQ plus a 2025 archive link). So there is no upcoming cycle to open soon; this is a completed event.

That matches what the submitter said on #148: "Currently closed, but adding to the list."

Title updated to TreeHacks 2026 — Feb 13–15, 2026 so the archive records which cycle it was. With the archive automation now on main, a closed listing renders into ARCHIVE.md instead of the README — verified it lands there correctly.

The merge conflict was only in assets/hackathons-banner.svg, resolved by regenerating it rather than by hand.