Deprecate legacy GitHub query config

[kitsuyui]

Summary

• Mark the legacy github.query config field as deprecated in code documentation.
• Emit a stderr warning when legacy configs still fall back to github.query.
• Add tests for the legacy fallback and the mutually exclusive query/queries error path.

Testing

• cargo fmt --all
• cargo check
• cargo test
• cargo fmt --all -- --check
• cargo clippy -- -D warnings
• cargo build
• actionlint -shellcheck=
• git diff --check

The reusable spellcheck workflow was not reproduced locally because this repository does not define a local command for it.

[github-actions]

Code Metrics Report

	main (b742c4a)	#840 (1a2926b)	+/-
Coverage	71.8%	72.3%	+0.4%
Code to Test Ratio	1:0.0	1:0.0	0.0
Test Execution Time	4m8s	5m23s	+1m15s

Details

  |                     | main (b742c4a) | #840 (1a2926b) |  +/-   |
  |---------------------|----------------|----------------|--------|
+ | Coverage            |          71.8% |          72.3% |  +0.4% |
  |   Files             |             18 |             18 |      0 |
  |   Lines             |           1949 |           1969 |    +20 |
+ |   Covered           |           1400 |           1424 |    +24 |
  | Code to Test Ratio  |          1:0.0 |          1:0.0 |    0.0 |
  |   Code              |           2528 |           2557 |    +29 |
  |   Test              |              0 |              0 |      0 |
- | Test Execution Time |           4m8s |          5m23s | +1m15s |

Code coverage of files in pull request scope (85.7% → 89.0%)

Files	Coverage	+/-	Status
src/config.rs	89.0%	+3.2%	modified

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Reported by octocov