fix: Allow ctrl+space to work as leader key

[razor-x]

What does this PR do?

Closes #3657 (which should have been re-opened)

Summary

• Normalize ParsedKey events so ctrl+space (NUL) and ctrl+_ map to named keys in one place, and simplify key parsing to rely on that normalization (packages/opencode/src/util/keybind.ts:13, packages/opencode/src/cli/cmd/tui/context/keybind.tsx:76).
• Add a focused unit test that fails pre-fix by asserting NUL maps to ctrl+space (packages/opencode/test/keybind.test.ts:190).

How did you verify your code works?

Tested locally and added test for key detection. Test verified failing before src changes.