feat: update e2e tests to properly work with async mode

[svarlamov]

Summary

Enables async mode (GIT_AI_ASYNC_MODE: "true") across all three CI workflows by starting a per-test/per-job daemon and configuring the wrapper to poll for authorship notes.

BATS e2e tests (tests/e2e/user-scenarios.bats):

• setup() creates an isolated TEST_DAEMON_HOME (overriding $HOME) with async-mode config, starts the daemon via git-ai bg run, and polls for Unix domain sockets to appear
• git() and git-ai() shell wrappers now pass daemon socket paths, trace2 config, force-TTY, and a 30s post-commit timeout
• teardown() sends bg shutdown, waits up to 2s, then kill -9 as fallback, restores $HOME, and cleans up both temp dirs
• New wait_for_note() helper polls for authorship notes (up to 20s) after non-commit operations like rebase where the wrapper doesn't auto-poll
• init.defaultBranch main set in global gitconfig before git init (required because isolated HOME loses the system default)

Reusable daemon helper scripts (scripts/nightly/):

• start-async-daemon.sh: creates isolated daemon home with async-mode config, probes /usr/bin/git and /usr/local/bin/git to avoid accidentally using the git-ai proxy symlink, starts daemon in background, polls for sockets (10s), exports all env vars to GITHUB_ENV for subsequent workflow steps
• stop-async-daemon.sh: graceful bg shutdown via control socket, 2s wait, force-kill fallback, daemon home cleanup

CI workflow changes:

• e2e-tests.yml: GIT_AI_ASYNC_MODE flipped to "true"
• install-scripts-local.yml: GIT_AI_ASYNC_MODE flipped to "true" with Start/Stop daemon steps around Unix test steps; Windows job overrides to "false" (no Unix-domain socket support)
• nightly-agent-integration.yml: GIT_AI_ASYNC_MODE flipped to "true" with Start/Stop daemon steps in both Tier 1 (hook wiring + synthetic checkpoint) and Tier 2 (live agent + attribution verification) jobs

Closes #839

Review & Testing Checklist for Human

• Daemon process survival across GitHub Actions steps: The daemon is started as a background process via source start-async-daemon.sh in one step and must stay alive for subsequent steps. The PID is persisted via GITHUB_ENV for the stop step. Verify background processes are not killed between steps on Ubuntu and macOS runners.
• Env var propagation to nightly test scripts: The existing scripts (test-synthetic-checkpoint.sh, verify-synthetic-attribution.sh, etc.) are not modified — they depend on GIT_AI_TEST_FORCE_TTY and GIT_AI_POST_COMMIT_TIMEOUT_MS being set in the environment via GITHUB_ENV. Confirm these are correctly inherited by the bash subprocesses that run the scripts.
• Real git path detection on macOS: start-async-daemon.sh probes /usr/bin/git then /usr/local/bin/git to avoid the proxy symlink. Verify this finds the correct git binary on macos-latest runners (Xcode CLT vs Homebrew).
• git_hooks_enabled: false in daemon config doesn't prevent authorship note generation — the daemon should use trace2 events rather than git hooks, but verify this is the correct code path in async mode.
• Recommended test plan: Trigger the install-scripts-local and nightly-agent-integration workflows manually on this branch (apply integration label to the PR) and verify all jobs pass. For install-scripts-local, check both Ubuntu and macOS Unix jobs succeed and Windows jobs still pass with sync mode.

Notes

• No Rust source changes — only BATS test infrastructure, CI workflows, and new shell helper scripts
• task lint and task format:check pass
• Windows jobs in install-scripts-local.yml remain on sync mode (GIT_AI_ASYNC_MODE: "false") because Unix-domain sockets are unavailable
• Commit operations rely on the wrapper's built-in post-commit polling (GIT_AI_TEST_FORCE_TTY=1 + GIT_AI_POST_COMMIT_TIMEOUT_MS=30000); wait_for_note() is only needed after rebase/squash operations in the BATS tests
• CI timeouts: daemon start 10s, post-commit polling 30s, wait_for_note 20s — confirm these are sufficient under load

Link to Devin session: https://app.devin.ai/sessions/85ba6a1442ec430b807e35f9d66c23dc
Requested by: @svarlamov

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