fix(cli): fail install when skill links fail

[sunjiajunsunjiajun]

Summary

• make rgit install return a non-zero exit code when skill symlinks, guidance writes, or installer subprocesses fail
• stop printing misleading success instructions and restart guidance after a failed install
• report failed skill linking with a failure mark in human-readable output
• add text, JSON, and guidance-failure regression coverage
• make the test suite portable on native Windows by using a Python no-op command and capability-gating real directory-symlink integration tests

Closes #34.

Reproduction and verification

Simulated the Windows environment where directory symlink creation fails with [WinError 1314] A required privilege is not held by the client.

Behavior	main (76f45b5)	This branch
CLI exit code	0	1
skill-link status	misleading ✓	✗
success instructions	printed	suppressed
restart advice	printed	suppressed
actionable Developer Mode hint	printed	printed

Issue-specific regression tests:

3 passed, 122 deselected

Full suite on native Windows with an isolated pytest temp root:

517 passed, 20 skipped in 71.77s

The additional skips are tests that require real directory-symlink capability; dedicated failure-path tests continue to verify the no-privilege behavior.

Review

Independent read-only review found no Critical, Important, or blocking Minor issues and assessed the change as ready to merge.