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/cli/config-manager/opencode-binary.ts

@@ -1,3 +1,4 @@
+import { extractSemverFromOutput } from "../../shared/extract-semver"
 import type { OpenCodeBinaryType } from "../../shared/opencode-config-dir-types"
 import { spawnWithWindowsHide } from "../../shared/spawn-with-windows-hide"
 import { initConfigContext } from "./config-context"
@@ -19,7 +20,7 @@ async function findOpenCodeBinaryWithVersion(): Promise<OpenCodeBinaryResult | n
       const output = await new Response(proc.stdout).text()
       await proc.exited
       if (proc.exitCode === 0) {
-        const version = output.trim()
+        const version = extractSemverFromOutput(output) ?? output.trim()
         initConfigContext(binary, version)
         return { binary, version }
       }