Add split-wezterm: split WezTerm pane into a rows x cols grid

Splits the current pane into evenly-sized grid using wezterm cli
split-pane. Supports optional command per pane, dryrun mode, and
auto-detects native or Flatpak WezTerm CLI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

specs/split-wezterm.spec.md

@@ -0,0 +1,89 @@
+# split-wezterm — OpenSpec
+
+## Purpose
+
+Split the current WezTerm pane into an evenly-sized grid of rows × columns, optionally running a command in each new pane.
+
+## Usage
+
+```
+split-wezterm <rows> <cols> [--] [command...]
+split-wezterm --help
+split-wezterm --dryrun <rows> <cols> [--] [command...]
+```
+
+### Arguments
+
+| Argument | Required | Description |
+|----------|----------|-------------|
+| `rows` | Yes | Number of rows (1-10) |
+| `cols` | Yes | Number of columns (1-10) |
+| `command...` | No | Command to run in each new pane. If omitted, panes open the default shell. |
+
+### Flags
+
+| Flag | Description |
+|------|-------------|
+| `--help`, `-h` | Show usage information |
+| `--dryrun`, `-n` | Preview the split-pane commands without executing them |
+
+## Behaviour
+
+1. Validate that `rows` and `cols` are integers between 1 and 10
+2. Detect the WezTerm CLI binary (`wezterm` or `flatpak run org.wezfurlong.wezterm`)
+3. Get the current pane ID from `$WEZTERM_PANE` or fall back to `wezterm cli list` to find the active pane
+4. Create the grid using the following algorithm:
+   a. **Create rows first:** Split the original pane vertically (rows-1) times. Each split uses `--bottom --percent P` where P is calculated to produce equal-height rows. The first split takes `(rows-1)/rows` as the bottom percent, the next takes `(rows-2)/(rows-1)`, etc.
+   b. **Collect row pane IDs:** The original pane becomes row 0. Each `split-pane --bottom` returns the new pane ID.
+   c. **Create columns:** For each row pane, split horizontally (cols-1) times using the same proportional math for equal-width columns.
+   d. **Run command:** If a command was specified, pass it as the program argument to each `split-pane` call. The first pane (original) doesn't get split — if a command is specified, use `wezterm cli send-text` to run it there.
+5. Output a summary: "Created {rows}x{cols} grid ({total} panes)"
+
+### Proportional splitting math
+
+To split a pane into N equal parts:
+- Split i (0-indexed, i=0..N-2): `percent = (N - 1 - i) * 100 / (N - i)`
+- This ensures each resulting section is 1/N of the original.
+
+Example for 3 splits: percent = 66, 50 (producing 33/33/33 split).
+
+## Dryrun behaviour
+
+In `--dryrun` mode, print each `wezterm cli split-pane` command that would be executed, prefixed with `[dryrun]`. Do not execute any commands. Still validate inputs and detect the CLI binary.
+
+Example:
+```
+[dryrun] wezterm cli split-pane --pane-id 0 --bottom --percent 66
+[dryrun] wezterm cli split-pane --pane-id 0 --bottom --percent 50
+[dryrun] wezterm cli split-pane --pane-id 0 --horizontal --percent 75
+...
+[dryrun] Would create 3x4 grid (12 panes)
+```
+
+## Edge cases
+
+- `1 1` — No splits needed. Print "Already a single pane, nothing to do." and exit 0.
+- `1 N` — Only horizontal splits (columns only, no row splits).
+- `N 1` — Only vertical splits (rows only, no column splits).
+- No WezTerm CLI available — exit 1 with error message.
+- `$WEZTERM_PANE` not set and no active pane found — exit 1 with error.
+- Non-integer or out-of-range arguments — exit 1 with usage hint.
+
+## Examples
+
+```bash
+# Split into 3 rows x 4 columns
+split-wezterm 3 4
+
+# Split into 2x2 grid, each pane running htop
+split-wezterm 2 2 -- htop
+
+# Split into 2x3 grid, each running claude with a profile
+split-wezterm 2 3 -- claude-with-profiles
+
+# Preview what would happen
+split-wezterm --dryrun 3 4
+
+# Single row, 3 columns
+split-wezterm 1 3
+```