fix(cli): Wire dialect flag through config merge for all commands

[mhiro2]

Summary

• Fix [render]/[inspect]/[export]/[doc]/[lint] dialect = "..." in the TOML config being silently ignored.
  clap's default_value = "auto" on --dialect meant the CLI always supplied a concrete value, so the
  args.dialect.or(config.dialect) merge never saw None and the file-level setting was shadowed.
• Switch those five commands' --dialect to Option<DialectArg> and resolve the effective dialect through
  the existing merge_*_args path, restoring the documented precedence (defaults → config → CLI). diff
  already worked this way and is unchanged.
• Document the key in docs/configuration.md and the full fixture, and add regression tests so the merge
  path can't silently regress again.

Changes

• 132ce4d : fix(cli): wire dialect flag through config merge for all commands
  • Make --dialect an Option<DialectArg> on render/inspect/export/doc/lint and merge it via
    args.dialect.or(self.<cmd>.dialect).unwrap_or_default(); carry the resolved value on MergedXxxConfig and
    feed merged.dialect.into() into input resolution.
  • Refresh help and deny_unknown_fields snapshots now that --dialect no longer advertises [default: auto] and the accepted field list includes dialect.
• be89a33 : docs(config): document dialect key and add merge regression tests
  • Add a dialect row to every command table in docs/configuration.md and populate
    fixtures/config/valid_full.toml / the config.rs module doc with the new key.
  • Add 10 regression tests (two per command) proving [<cmd>] dialect = "..." is honored when --dialect
    is unset and that the CLI still wins when both are set.

[github-actions]

Code Metrics Report

	main (e359a57)	#83 (25b6c0e)	+/-
Coverage	94.6%	94.6%	+0.0%
Test Execution Time	3m18s	1m26s	-1m52s

Details

  |                     | main (e359a57) | #83 (25b6c0e) |  +/-   |
  |---------------------|----------------|---------------|--------|
+ | Coverage            |          94.6% |         94.6% |  +0.0% |
  |   Files             |             77 |            77 |      0 |
  |   Lines             |          33588 |         33774 |   +186 |
+ |   Covered           |          31795 |         31981 |   +186 |
+ | Test Execution Time |          3m18s |         1m26s | -1m52s |

Code coverage of files in pull request scope (94.6% → 95.2%)

Files	Coverage	+/-	Status
crates/relune-cli/src/cli.rs	88.0%	0.0%	modified
crates/relune-cli/src/commands/doc.rs	76.1%	0.0%	modified
crates/relune-cli/src/commands/export.rs	92.1%	0.0%	modified
crates/relune-cli/src/commands/inspect.rs	100.0%	0.0%	modified
crates/relune-cli/src/commands/lint.rs	77.0%	0.0%	modified
crates/relune-cli/src/commands/render.rs	98.4%	0.0%	modified
crates/relune-cli/src/config.rs	96.4%	+0.6%	modified

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Reported by octocov