feat: run mo in background by default

[k1LoW]

Summary

• Run mo server as a background process by default so the shell returns immediately after startup. This makes mo easy to incorporate into scripts, tool chains, and LLM-driven workflows.
• Add --foreground flag to keep the server in the foreground when needed (e.g. debugging, make dev).
• Add --shutdown user-facing stderr messages alongside existing slog for tryAddToExisting and doShutdown.
• Extract server.WriteRestoreFile() to deduplicate temp file writing between ExportState and background startup.
• Extract openBrowser() helper to deduplicate browser-open logic.
• Use os.Executable() + Setsid (setSysProcAttr) for robust process detachment on Unix.

[github-actions]

Code Metrics Report

	main (28e60d2)	#50 (be8d871)	+/-
Coverage	41.1%	40.3%	-0.9%
Code to Test Ratio	1:0.4	1:0.4	-0.1
Test Execution Time	34s	33s	-1s

Details

  |                     | main (28e60d2) | #50 (be8d871) |  +/-  |
  |---------------------|----------------|---------------|-------|
- | Coverage            |          41.1% |         40.3% | -0.9% |
  |   Files             |             25 |            26 |    +1 |
  |   Lines             |           2459 |          2512 |   +53 |
  |   Covered           |           1013 |          1013 |     0 |
- | Code to Test Ratio  |          1:0.4 |         1:0.4 |  -0.1 |
  |   Code              |           2968 |          2984 |   +16 |
  |   Test              |           1326 |          1326 |     0 |
+ | Test Execution Time |            34s |           33s |   -1s |

Code coverage of files in pull request scope (19.6% → 18.3%)

Files	Coverage	+/-	Status
cmd/root.go	0.0%	0.0%	modified
cmd/sysprocattr_unix.go	0.0%	0.0%	added
internal/server/server.go	27.9%	-0.2%	modified

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Reported by octocov