feat(access): add --pidfile support for access command (fixes #723)

[MauroDruwel]

Summary

Fixes #723

Adds --pidfile command-line flag (and TUNNEL_PIDFILE environment variable) support to cloudflared access and cloudflared access tcp (including aliases ssh, rdp, and smb).

Problem & Motivation

When running cloudflared access tcp as a background process or forwarder in automation scripts, Taskfiles, or CI pipelines (e.g. for forwarding SOCKS5, Kubernetes API, or Vault access), users currently have to resort to fragile pgrep / grep pipelines to find and terminate the process during cleanup traps.

cloudflared tunnel already provides a --pidfile flag. Bringing parity to cloudflared access enables clean process management and reliable shutdown (e.g., pkill -F .cloudflared.pid or kill -15 $(cat .cloudflared.pid)).

Changes

1. Shared PID Helper: Extracted writePidFile logic into cmd/cloudflared/cliutil/pidfile.go (cliutil.WritePidFile) supporting ~ homedir expansion, path cleaning, and structured error logging. Refactored cmd/cloudflared/tunnel/cmd.go to use this shared utility to eliminate duplicate code.
2. Flag Registration:
   • Added cfdflags.Pidfile constant in cmd/cloudflared/flags/flags.go.
   • Registered --pidfile with TUNNEL_PIDFILE env var on cloudflared access and the tcp subcommand in cmd/cloudflared/access/cmd.go.
3. Execution Hook: In cmd/cloudflared/access/carrier.go, cliutil.WritePidFile is called for both background forwarding listener mode (--url) and client stream proxy mode.

Testing Plan

• Unit Tests:
  • Added cmd/cloudflared/cliutil/pidfile_test.go verifying normal file creation with os.Getpid(), homedir ~ expansion, and graceful error handling on invalid paths.
  • Added cmd/cloudflared/access/pidfile_test.go verifying --pidfile and TUNNEL_PIDFILE registration across access and access tcp.
  • Ran go test -count=1 on cliutil, access, tunnel, and carrier packages — all passed.
• End-to-End Verification:
  • Verified live process execution: cloudflared access tcp --hostname ... --url 127.0.0.1:23456 --pidfile /tmp/test.pid created the file with the exact running PID.
  • Verified parent flag invocation: cloudflared access --pidfile /tmp/test.pid tcp ....
• Linters:
  • gofmt -s -l: clean (0 issues).
  • go vet ./cmd/cloudflared/...: clean (0 issues).

[MauroDruwel]

Closing as duplicate of #1599.