feat(widgets): add editable label prefix to text-label widgets

[tough-griff]

Closes #600

Adds a per-widget label override for widgets that show a text label (Model: , Ctx: , In: , …). Select the widget in the line editor and press b (la(b)el…).

Changes

• Storage: the override is saved as metadata.label and used exactly as typed. An empty string removes the label, and a value equal to the default isn't stored, so untouched items stay minimal. No schema change or migration.
• Rendering: formatRawOrLabeledValue now uses the override, so every widget that already went through it gets the feature from one change. Widgets that built their label inline now use the helper too: Model, Session ID, Account Email, Context Length/Window/Bar, Memory Usage, Output Style, Session Clock/Name/Cost, Thinking Effort, Terminal Width, Git Staged/Unstaged/Untracked Files, and Current Working Dir (its optional glyph still comes before the label).
• Opt-in: widgets declare a new optional Widget.getLabelPrefix(item), which returns their default label for the item's current mode (Context % gives Ctx Used: or Ctx Left: ). The usage and speed families get theirs from their shared config.
• One override for every mode: widgets with more than one label variant (Block: vs Block before a bar) use the same override in all modes.
• TUI:
  • The keybind is added centrally in ItemsEditor, the same way (h)ide… is, and is hidden while raw value is on.
  • It opens a new LabelEditor. The value is shown in quotes so trailing spaces are visible, and Tab restores the default.
  • Rows show (label: "M ") while an override is set.
  • l would be the natural key, but the reset timers already use it for (l)ocale.
• Shared text cursor: CustomText's grapheme-aware cursor code is moved into useTextCursor (src/widgets/shared/text-cursor.ts), which the label editor and Custom Text now both use. This also makes Ctrl+←/→ jump to the start/end as Custom Text's hint says. The old checks compared input to 'ArrowLeft', which Ink never sends, and ran after the plain-arrow branch, so they never matched.
• Out of scope: glyph widgets keep (g)lyph. Widgets whose prefix is really part of the value (Version, Git PR, Sandbox/Voice/Remote Control status, Git Is Fork, Link) aren't changed.
• Docs: docs/USAGE.md covers the keybind in the raw value section and in the editor keybind list.

Testing

• Unit tests for the label helper and the text cursor (grapheme movement/deletion with emoji, edges, Ctrl jumps, cursor rendering)
• Override cases for Model and Session Usage (text and slider modes), plus Context %'s mode-dependent default
• A registry-wide test that no label widget binds b itself in any mode, next to the existing h check
• An end-to-end ItemsEditor test covering b → edit → save → (label: "M "), with the keybind and row tag hidden in raw value mode
• bun test and bun run lint pass
• Manually checked in the TUI and with piped input:

$ echo '{"model":{"id":"claude-sonnet-4-5","display_name":"Sonnet 4.5"}}' | bun run src/ccstatusline.ts --config settings.json
M Sonnet 4.5 | Model: Sonnet 4.5 | 80

settings.json

{
  "version": 4,
  "lines": [
    [
      { "id": "1", "type": "model", "metadata": { "label": "M " } },
      { "id": "2", "type": "separator" },
      { "id": "3", "type": "model" },
      { "id": "4", "type": "separator" },
      { "id": "5", "type": "terminal-width", "metadata": { "label": "" } }
    ]
  ]
}

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