feat(tui): add configurable readline-style text transformations

[aspiers]

Summary

Adds missing Emacs/Readline-style text editing shortcuts to the TUI (terminal) prompt input, improving text editing efficiency in the terminal.

Changes

New shortcuts

Shortcut	Action
alt+u	Uppercase word from cursor
alt+l	Lowercase word from cursor
alt+c	Capitalize word from cursor
ctrl+y	Yank (paste) last killed text
ctrl+t	Transpose characters (requires rebind)

User-facing behavior

Case transformations (alt+u, alt+l, alt+c)

• When cursor is on a word character: transforms the word starting from cursor position
• When cursor is on whitespace: transforms the next word
• When no next word exists: transforms the previous word
• With text selection: transforms the selected text
• After transformation: cursor moves to end of the transformed word

Kill ring support (ctrl+y)

• The ctrl+k, ctrl+u, ctrl+w, and alt+d delete commands now save deleted text to a kill buffer
• ctrl+y inserts the contents of the kill buffer at the current cursor position
• Simple single-entry buffer (full kill ring can be added later)

Transpose characters (ctrl+t)

• Swaps the character under the cursor with the character immediately preceding it
• At beginning of line: swaps first two characters
• At end of line: swaps last two characters
• Cursor advances one position to the right (unless already at end of line)

Configuration

All new shortcuts are configurable via opencode.json:

{
  "keybinds": {
    "input_lowercase_word": "alt+l",
    "input_uppercase_word": "alt+u",
    "input_capitalize_word": "alt+c",
    "input_yank": "ctrl+y",
    "input_transpose_characters": "ctrl+t"
  }
}

Note on ctrl+t conflict:
By default, ctrl+t is bound to variant_cycle (cycle model variants). To use ctrl+t for transpose characters, you can rebind, e.g.:

{
  "keybinds": {
    "variant_cycle": "<leader>v",
    "input_transpose_characters": "ctrl+t"
  }
}

Technical details

• TUI uses OpenTUI's TextareaRenderable which has limited built-in actions
• Implementation adds helper functions for word boundary detection matching Readline behavior
• Tests added for word boundary detection and transformation functions

Files

• packages/opencode/src/config/config.ts - Added keybind schema entries
• packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx - Implemented shortcuts
• packages/web/src/content/docs/keybinds.mdx - Updated documentation
• packages/sdk/js/src/v2/gen/types.gen.ts - Regenerated from schema
• packages/opencode/test/tui/text-transform.test.ts - Added 17 tests

[aspiers]

I'm guessing the test failures are due to #6674, rather than anything to do with this PR.

[aspiers]

Tests passing after rebase.