test(coverage): expand test suite coverage to 80% and harden error boundaries

[JacksonFergusonDev]

Summary

This PR significantly expands the Tier 1 test suite from 99 tests (65.74% coverage) to 156 tests (79.64% branch coverage).

Additionally, the test coverage threshold (fail_under) in pyproject.toml has been raised from 60 to 75 to protect against future coverage regression.

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Detailed Changes by Module

1. test_git_wrapper.py (+7 tests, 86% coverage)

• Validation & Error Propagation: Verifies GitRepo raises ValueError on non-repositories and translates CalledProcessError to RuntimeError with stderr context.
• Plumbing Safety: Tests atomic ref update guards with old_oid, re-raising on update_ref failure, and commit_tree error handling.
• Branch Operations: Added tests for merge_squash no-op on empty branches and checkout force flag handling.

2. test_daemon.py (+13 tests, 81% coverage)

• Skip & Push Guards: Tested _should_skip under critical battery, high system load, missing paths, and paused repos.
• Eco-Mode & Connectivity: Tested _attempt_push skipping in eco-mode and when remote endpoints are unreachable.
• Reachability & Locks: Added tests for SSH/HTTPS URL parsing in get_remote_host, TCP reachability in is_remote_reachable, and operational/stale lock detection in is_repo_busy.
• Maintenance Lifecycle: Verified weekly backup pruning interval enforcement (7 days) in run_maintenance.

3. test_ops.py (+15 tests, 82% coverage)

• Maintenance: Added tests for prune_backups deleting stale backup references and running git gc --auto.
• Ignore Management: Added tests for add_ignore appending to .gitignore, skipping duplicates, respecting manage_gitignore=False, and prompting to untrack matching files.
• Sync & Restore Safety: Added tests for sync_session when disabled, when up to date, and on user decline. Added restore_file failure exits and force overwrite handling.

4. test_cli.py (+15 tests, 76% coverage)

• Repo Management: Added tests for list_repos (missing, paused, empty registry) and unregister_repo (clean removal, unlisted path warning, empty registry).
• Diagnostics & Status: Added tests for show_diff outside a git repo and with untracked files, _check_repo_health paused states and missing backups, and show_status on unregistered repos.
• State Control: Added tests for set_pause_state creating and unlinking the pulsar_paused sentinel.

5. test_system.py (+5 tests, 73% coverage)

• Telemetry & Monitoring: Added tests for is_under_load CPU load ratio thresholding, pmset battery parsing on macOS, sysfs battery capacity/status parsing on Linux, and the get_system factory function.

6. Configuration & Documentation

• pyproject.toml: Raised fail_under to 75.
• tests/README.md: Updated documentation to reflect the expanded testing matrix.

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Verification

• just ci ran locally: all 156 unit tests passed, all linting/formatting/typechecks passed, and Tier 2 distributed sandbox passed cleanly.