refactor: apply minimal tfcode branding

- Rename packages/opencode → packages/tfcode (directory only)
- Rename bin/opencode → bin/tfcode (CLI binary)
- Rename .opencode → .tfcode (config directory)
- Update package.json name and bin field
- Update config directory path references (.tfcode)
- Keep internal code references as 'opencode' for easy upstream sync
- Keep @opencode-ai/* workspace package names

This minimal branding approach allows clean merges from upstream
opencode repository while providing tfcode branding for users.

packages/tfcode/src/shell/shell.ts

@@ -0,0 +1,73 @@
+import { Flag } from "@/flag/flag"
+import { lazy } from "@/util/lazy"
+import { Filesystem } from "@/util/filesystem"
+import { which } from "@/util/which"
+import path from "path"
+import { spawn, type ChildProcess } from "child_process"
+import { setTimeout as sleep } from "node:timers/promises"
+
+const SIGKILL_TIMEOUT_MS = 200
+
+export namespace Shell {
+  export async function killTree(proc: ChildProcess, opts?: { exited?: () => boolean }): Promise<void> {
+    const pid = proc.pid
+    if (!pid || opts?.exited?.()) return
+
+    if (process.platform === "win32") {
+      await new Promise<void>((resolve) => {
+        const killer = spawn("taskkill", ["/pid", String(pid), "/f", "/t"], {
+          stdio: "ignore",
+          windowsHide: true,
+        })
+        killer.once("exit", () => resolve())
+        killer.once("error", () => resolve())
+      })
+      return
+    }
+
+    try {
+      process.kill(-pid, "SIGTERM")
+      await sleep(SIGKILL_TIMEOUT_MS)
+      if (!opts?.exited?.()) {
+        process.kill(-pid, "SIGKILL")
+      }
+    } catch (_e) {
+      proc.kill("SIGTERM")
+      await sleep(SIGKILL_TIMEOUT_MS)
+      if (!opts?.exited?.()) {
+        proc.kill("SIGKILL")
+      }
+    }
+  }
+  const BLACKLIST = new Set(["fish", "nu"])
+
+  function fallback() {
+    if (process.platform === "win32") {
+      if (Flag.OPENCODE_GIT_BASH_PATH) return Flag.OPENCODE_GIT_BASH_PATH
+      const git = which("git")
+      if (git) {
+        // git.exe is typically at: C:\Program Files\Git\cmd\git.exe
+        // bash.exe is at: C:\Program Files\Git\bin\bash.exe
+        const bash = path.join(git, "..", "..", "bin", "bash.exe")
+        if (Filesystem.stat(bash)?.size) return bash
+      }
+      return process.env.COMSPEC || "cmd.exe"
+    }
+    if (process.platform === "darwin") return "/bin/zsh"
+    const bash = which("bash")
+    if (bash) return bash
+    return "/bin/sh"
+  }
+
+  export const preferred = lazy(() => {
+    const s = process.env.SHELL
+    if (s) return s
+    return fallback()
+  })
+
+  export const acceptable = lazy(() => {
+    const s = process.env.SHELL
+    if (s && !BLACKLIST.has(process.platform === "win32" ? path.win32.basename(s) : path.basename(s))) return s
+    return fallback()
+  })
+}