fix: resolve nightly agent test failures for droid and opencode

[Siddhant-K-code]

Summary

Fixes the persistent nightly failures in "Tier 1: droid latest" and "Tier 1: opencode latest" jobs (failing daily since at least 2026-06-22). Closes #1723.

Droid — settings.json not found: DroidInstaller::check_hooks only checked ~/.factory dir existence. When droid CLI installs to ~/.local/bin without creating ~/.factory/, check_hooks returned tool_installed: false and git-ai install skipped hook setup entirely.

 fn check_hooks(&self, _params: &HookInstallerParams) -> Result<HookCheckResult, GitAiError> {
+    let has_binary = binary_exists("droid");
     let has_dotfiles = home_dir().join(".factory").exists();
-    if !has_dotfiles {
+    if !has_binary && !has_dotfiles {
         return Ok(HookCheckResult { tool_installed: false, ... });

This matches the pattern already used by GeminiInstaller and OpenCodeInstaller.

OpenCode — command not found (exit 127): opencode-ai@next renamed its binary from opencode to opencode2. The workflow hardcoded opencode --version in verification.

Fix: resolve binary names dynamically via npm view <pkg> bin --json at matrix build time, pass matrix.binary_name through, and use it in:

• "Verify agent binary is present" step (uses ${{ matrix.binary_name }} instead of hardcoded case)
• verify-hook-wiring.sh (accepts optional binary_name arg)
• test-live-agent.sh (all five agents now use $BINARY_NAME consistently)

Also updated OpenCodeInstaller::check_hooks to detect opencode2 via binary_exists("opencode") || binary_exists("opencode2") and added "opencode2" to process_names().

Tests: Added 8 new #[serial] unit tests covering detection logic for both installers — binary-on-PATH without dotfiles, no-binary-no-dotfiles, dotfiles-only, and full install flows from binary detection. Uses with_temp_home + with_fake_binary_on_path helpers following the existing gemini.rs pattern.

Cleanup: Removed dead headless_cmd binary-name replacement computation (pre-existing dead field; no longer adding unnecessary .replace() on top).

Link to Devin session: https://app.devin.ai/sessions/593476fb73c54d249654129305965de2
Requested by: @Siddhant-K-code

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