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/doctor/checks/system-binary.test.ts

@@ -1,7 +1,7 @@
 /// <reference types="bun-types" />
 
 import { describe, expect, it } from "bun:test"
-import { extractSemverFromOutput } from "./system-binary"
+import { extractSemverFromOutput } from "../../../shared/extract-semver"
 
 describe("extractSemverFromOutput", () => {
   describe("#given clean version output #when extractSemverFromOutput #then returns the semver token", () => {

src/cli/doctor/checks/system-binary.ts

@@ -1,10 +1,13 @@
 import { existsSync } from "node:fs"
 import { homedir } from "node:os"
 import { join } from "node:path"
+import { extractSemverFromOutput } from "../../../shared/extract-semver"
 import { spawnWithTimeout } from "../spawn-with-timeout"
 
 import { OPENCODE_BINARIES } from "../constants"
 
+export { extractSemverFromOutput }
+
 const WINDOWS_EXECUTABLE_EXTS = [".exe", ".cmd", ".bat", ".ps1"]
 
 export interface OpenCodeBinaryInfo {
@@ -105,17 +108,6 @@ export async function findOpenCodeBinary(): Promise<OpenCodeBinaryInfo | null> {
   return findDesktopBinary()
 }
 
-export function extractSemverFromOutput(output: string): string | null {
-  const trimmed = output.trim()
-  if (!trimmed) return null
-  // Match a semver-shaped token, allowing optional `v` prefix and `-prerelease+build` suffix.
-  // 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
-}
-
 export async function getOpenCodeVersion(
   binaryPath: string,
   platform: NodeJS.Platform = process.platform