fix(cli): exit non-zero when miner post / miner check report success: false

[ebios-star]

Summary

`gitt miner post` and `gitt miner check` already encode their logical outcome in the JSON `success` field (`accepted_count > 0` / `valid_count > 0`), but the process exit code stayed `0` even when every validator rejected the broadcast or no validator had a valid stored PAT. Shell pipelines and CI steps that ran `gitt miner post && next-step` or used the exit code as a health-check signal silently treated total failure as success.

After this change both commands `sys.exit(1)` when their respective count is zero — matching the JSON `success` field and the family already fixed for `gitt admin` / `gitt vote` / `gitt harvest` in #707 and `gitt issues list --id` / miner-check JSON aggregate counts in #724.

The fix is minimal: 6 lines in post.py and 6 lines in check.py, each guarded so the success exit code stays `0`.

Related Issues

Closes #841 (`gitt miner post`)
Closes #842 (`gitt miner check`)

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• New `TestMinerPostExitCodeOnZeroAccepted` — 2 cases (zero-accepted exits 1, at-least-one-accepted exits 0).
• New `TestMinerCheckExitCodeOnZeroValid` — 2 cases (zero-valid exits 1, at-least-one-valid exits 0).
• The harmless "coroutine never awaited" RuntimeWarning that fires because we patch `asyncio.run` is filtered at class scope.
• `uv run --extra dev pytest tests/` — 647 tests pass (643 baseline + 4 new).

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[ebios-star]

@anderdc @LandynDev — friendly ping for review when you have a moment.

Closes #841 and #842. gitt miner post and gitt miner check already declared success: false in their JSON output when no validator accepted / had a valid PAT, but the process exit code stayed 0 — so any shell pipeline using miner post && next-step or miner check as a health-check signal silently treated total failure as success.

Same family as #707 (your merged gitt admin/vote/harvest exit-code fix) and #724 (which fixed the JSON aggregate counts on miner check but left the exit-code half open). 6 lines per command + 4 new regression tests across two test classes.

Happy to rebase if needed.