fix(installer): apply extractSemverFromOutput to opencode-binary version probe

The same output.trim() bug fixed in PR #3909 for doctor exists in the
installer's opencode-binary.ts. Without this fix, `bunx oh-my-opencode
install` would store polluted Electron stdout (e.g., `00:24:25.202 >
app starting { version: '1.14.33', packaged: true }`) as the OpenCode
version in config, breaking downstream version-dependent logic.

- Extract extractSemverFromOutput to src/shared/extract-semver.ts
  (precedent: spawn-with-windows-hide is in shared because used by
  both doctor and installer)
- src/cli/doctor/checks/system-binary.ts now imports from shared and
  re-exports for backward compat
- src/cli/config-manager/opencode-binary.ts uses the shared helper
  with `?? output.trim()` fallback to preserve legacy behavior on
  non-semver-shaped successful outputs (e.g., custom builds)
- Add 4 installer regression tests covering: clean semver, polluted
  Electron stdout (regression for #3765 installer caller), fallback
  for non-semver, null when no binary on PATH

Refs #3765

src/shared/extract-semver.ts

@@ -0,0 +1,9 @@
+export function extractSemverFromOutput(output: string): string | null {
+  const trimmed = output.trim()
+  if (!trimmed) return null
+  // The negative lookbehind `(?<![\d:])` prevents matching the milliseconds segment of timestamps
+  // like `00:24:25.202` that the Electron-based OpenCode binary leaks into stdout.
+  const semverPattern = /(?<![\d:])v?(\d+\.\d+\.\d+(?:[-+][\w.]+)*)/
+  const match = trimmed.match(semverPattern)
+  return match?.[1] ?? null
+}