Extract `run_interruptible()` from bridge modules to eliminate the 3× duplicated subprocess poll-wait-shutdown loop

[arndvs]

Problem

The bridge's shutdown-aware subprocess execution pattern — start a Popen in a new session, poll it in a tight loop checking both a shutdown flag and a wall-clock deadline, capture stdout/stderr to temp files, and call terminate_process_group on any failure path — is implemented independently in three modules:

Module	Function	Lines of loop logic
bridge/workspace.py	_run_git()	~50
bridge/github.py	mint_token()	~40
bridge/worker.py	_wait_for_subprocess() + _run_subprocess()	~30

Issue #255 extracted the termination tail (terminate_process_group) but left the poll-wait loop — the harder, more bug-prone part — duplicated. Each copy uses the same control flow with different variable names and error wrappers. A change to poll interval, deadline semantics, or shutdown behavior must be applied three times.

Proposed solution

Extract a single run_interruptible() helper into bridge/subprocess_helpers.py (where terminate_process_group already lives) with this signature:

def run_interruptible(
    cmd: list[str],
    *,
    env: dict[str, str],
    timeout: float,
    shutdown_check: Callable[[], bool] | None = None,
    capture_output: bool = True,
    cwd: str | None = None,
    start_new_session: bool = True,
) -> subprocess.CompletedProcess:

When shutdown_check is None, delegate to subprocess.run (preserving the fast path all three callers already have). When provided, implement the poll loop once with terminate_process_group on all failure paths.

Each caller replaces its inline loop with a one-line call and keeps only its domain-specific error wrapping (WorkspaceError, GitHubError, RuntimeError).

Success criteria

• run_interruptible lives in bridge/subprocess_helpers.py
• workspace._run_git, github.mint_token, and worker._run_subprocess delegate to it
• Unit tests cover: happy path, timeout, shutdown interruption, non-zero exit
• Existing test/python/test_bridge_subprocess_helpers.py extended (not a new file)
• No behavioral change to worker, webhook, or workspace lifecycle

Out of scope

• Consolidating httpx.Client usage (Consolidate per-call GitHub API clients in the bridge into a session-scoped GitHubSession to eliminate redundant TLS handshakes and enable centralized rate-limit handling #292 tracks that separately)
• Changing the subprocess poll interval or timeout defaults
• Async subprocess support

ADR alignment

No ADR governs subprocess execution patterns. This proposal follows the project's stated preference: "Prefer removing or simplifying code over adding abstractions" (CONTEXT.md) — it removes ~120 lines of duplicated control flow and replaces them with one implementation.