fix: detect terminal width on Windows via parent console

[jcmecham]

Summary

On Windows, getTerminalWidth() always returned null, so flex separators collapsed and long-line truncation was disabled. Claude Code spawns the statusline with piped stdio and no controlling PTY, so process.stdout.columns is undefined; there's no columns field in the statusline JSON and no COLUMNS env var to fall back on; and the existing Unix ps/stty probe doesn't apply on Windows.

This PR makes width detection work on Windows by walking up the process tree to the claude.exe ancestor (which owns the real console), attaching to that console via a PowerShell P/Invoke helper, and reading the width from the Win32 console API.

Performance:

The probe runs synchronously inside ccstatusline (not on Claude Code's main thread, so the REPL is unaffected). Cold call: ~5s (PowerShell startup + Add-Type JIT). The result is cached at %TEMP%/ccstatusline-win-width-cache.json with a 60s TTL, so subsequent status-line renders read from disk in ~5ms. Trade-off: up to 60s of resize lag before the cache refreshes — acceptable for a status line.

Changes

File	Change
src/utils/terminal.ts	Windows branch added to getTerminalWidth(). Shells out to the probe, caches the result at %TEMP%/ccstatusline-win-width-<ppid>.json with a 60s TTL, returns null on any failure (no regression). In TUI mode (stdout is a real TTY) it returns process.stdout.columns directly and skips the probe. Non-Windows paths untouched.
scripts/windows-width-probe.ps1 (new)	PowerShell + C# P/Invoke probe. Walks PPIDs with CreateToolhelp32Snapshot, AttachConsoles to the owning process, reads CONSOLE_SCREEN_BUFFER_INFO.dwSize.X. Invoked via spawnSync -EncodedCommand to avoid cmd.exe arg truncation.
src/tui/components/ItemsEditor.tsx	Removed the "Terminal width detection is currently unavailable in your environment" warning — it existed to explain the broken Windows behavior this PR resolves.
src/utils/__tests__/terminal.test.ts	New tests for probe parsing, failure modes, and cache-hit behavior. Existing Unix tests now mock process.platform explicitly so they pass on Windows dev boxes.
package.json	build script copies the probe into dist/ so the published package ships it.

Test plan

• bun run vitest run src/utils/__tests__/terminal.test.ts — 12 tests pass.
• bun run lint — clean for touched files.
• Manual: bun run build, point statusLine.command at dist/ccstatusline.js, add a Flex Separator, resize Windows Terminal — line spans full width, reflows on resize within the 60s TTL.
• Manual: open the Items Editor in the TUI and hold down-arrow — instant response, no multi-second lag.