feat(editor): add opt-in vi-style modal editing

[laurigates]

Summary

This implements the vi-style editing requested in #80 as an opt-in feature
for the query editor. It is disabled by default, so the existing
emacs-style keybindings are completely unchanged for current users. Enable it
with:

[editor.vi]
enable = true

When enabled, the editor starts in NORMAL mode and the prefix (❮❮ by
default, configurable via normal_prefix) indicates the current mode. Press
i/a/I/A to enter INSERT mode — where typing, Tab completion, and the
configured keybinds all work exactly as before — and Esc to return to NORMAL
mode.

What's supported (NORMAL mode)

Category	Keys
Motions	h l 0 $ ^ w W b B e E f{char} F{char} t{char} T{char} gg G
Insert	i I a A
Operators	d / c / y + motion, and linewise dd cc yy
Edits	x X D C s S r{char} ~
Paste	p P (from the last delete/yank)
Counts	prefix a motion or operator, e.g. 3w, d2w, 5x
Undo / redo	u / Ctrl+R

The cw/cW quirk (acts like ce/cE on a non-blank) is implemented.

Design

• The modal logic lives in a new self-contained src/vi.rs module, written as
  a pure transformation over (text, cursor) that returns an Outcome the
  query editor applies. This keeps it decoupled from the promkit text widget
  and fully unit-testable — 29 tests cover the motions, operators, counts,
  find, paste, and edge cases (empty buffer, bounds).
• NORMAL-mode keys route through the engine; INSERT mode falls through to the
  existing text-editing path, so completion and the emacs keybinds are reused
  rather than reimplemented.
• [editor.vi] is #[serde(default)], so configuration files written before
  this change continue to load (covered by a test).

Scope / limitations

The query buffer is single-line, so linewise commands (dd, cc, yy) act on
the whole line, and o/O/.-repeat/multi-line motions are intentionally out
of scope. Happy to adjust the keymap, defaults, or scope to fit your
preferences — this is meant as a starting point for #80.

Testing

Mirrors CI (.github/workflows/ci.yml):

• cargo fmt --all -- --check — clean
• cargo clippy — clean
• cargo test — all green (29 new vi tests + 2 new config tests)
• cargo build --bins / --examples — ok

Addresses #80.

[laurigates]

Related PRs (no strict merge order) — #118 overlaps two sibling PRs textually: #116 (completion after pipes) via src/query_editor.rs / src/main.rs, and #119 (editor keybinds) via default.toml. Independent in intent; whichever merges first, the others need a quick rebase — no required order.