Revert field name and restore original config format

[mowi12]

Related Issue

Closes #64

Changes

• Rename 'discovery.root' back to 'discovery.root_path'
• Restore original config element order (root_path first, cache_ttl_hours second)
• Remove unnecessary single quotes from path values

How to Test

• Checkout new version
• Run mr-manager and save unchanged configs in editor
• See old config style

Checklist

Either tick the items or cross out those that do not apply (using example text) and provide a brief explanation why they do not apply.

Author

• I linked the issue that this PR closes.
• I tested the changes locally.
• I updated the documentation (if needed).
• I added/updated tests (if needed).

[github-actions]

🚀 Performance Benchmark Results

mr-manager startup benchmark summary
created_at: 2026-05-10T17:44:53.816256+00:00
scan_root: /home/runner/work/_temp/benchmark-fixtures
configured_runs_per_start_type: 10

Version references:
- v0.0.1: v0.0.1
- v0.0.2: v0.0.2
- v0.0.3: v0.0.3
- v1.0.0: v1.0.0
- fix/64-bug-configuration-field-renamed-to-root-and-order-changed-in-v100 (current): 933dfa90aac7

Per-version timings:
version                                        start  runs  mean_ms  median_ms  p95_ms  avg_repos
-------------------------------------------------------------------------------------------------
v0.0.1                                         cold     10     3.12       3.09    3.30       50.0
v0.0.2                                         cold     10     3.43       3.44    3.51       50.0
v0.0.2                                         warm     10     0.35       0.35    0.39       50.0
v0.0.3                                         cold     10     3.53       3.54    3.58       50.0
v0.0.3                                         warm     10     0.38       0.39    0.42       50.0
v1.0.0                                         cold     10     3.35       3.34    3.46       50.0
v1.0.0                                         warm     10     0.17       0.16    0.20       50.0
fix/64-bug-configuration-field-renamed-to-roo  cold     10     3.32       3.34    3.37       50.0
fix/64-bug-configuration-field-renamed-to-roo  warm     10     0.15       0.15    0.18       50.0

Warm-vs-cold speedup:
- v0.0.2: 9.73x faster warm start (89.7% less startup time)
- v0.0.3: 9.18x faster warm start (89.1% less startup time)
- v1.0.0: 19.60x faster warm start (94.9% less startup time)
- fix/64-bug-configuration-field-renamed-to-root-and-order-changed-in-v100 (current): 21.79x faster warm start (95.4% less startup time)```
<!-- thollander/actions-comment-pull-request "benchmark_results" -->

[Copilot]

Pull request overview

This PR addresses #64 by reverting a breaking change in the user config format, aiming to restore the pre-v1.0.0 key name and output style for ~/.config/mr-manager/config.yaml.

Changes:

• Switch config loading back to discovery.root_path (from discovery.root).
• Update config saving to emit root_path first and cache_ttl_hours second.
• Remove forced single-quoting of the saved root_path value.

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