fix(core): name the config file in language entry errors

[ElbertePlinio]

Closes #43

The unknown-language check ran on the merged config, after the user and repo files were combined, so its error could not say which file held the typo. It now runs per file inside validate_language_keys in crates/core/src/config.rs, and validate_languages is gone. The non-object languages.<name> message gains the same in <file> part. The language name is checked before the entry's shape, so {"languages":{"cobol":1}} reports the unknown language instead of a shape error, and {"languages":{"cobol":{"limits":{"bogus":1}}}} reports languages.cobol instead of the nested key.

Requirement	Verified by	Scenario	Result	Evidence
Non-object languages.<name> names the config file	automated test + CLI QA	{"go":1}, {"go":null}, non-object entry in the user config	pass, languages.go in <file> must be an object, exit 2	non_object_language_entry_names_config_path; QA 01, 02, 07
Unknown language names the config file	automated test + CLI QA	cobol in repo config; cobol in user config with a valid repo config; Go (case); empty name; unknown name with malformed entry or bad limits	pass, unknown config key `languages.<name>` in <file>, exit 2, and the user-config case names the user file	unknown_language_names_config_path; QA 03 to 06, 08, 09
A test covers the non-object language entry	automated test	exact message with the temp config path	pass	non_object_language_entry_names_config_path
Other config errors and valid configs unchanged	automated test + CLI QA	all seven languages, empty languages, non-object languages, per-language limit typo, valid user plus repo configs	byte-identical to main, including exit codes	existing config tests; QA 10 to 14
Tests, lint, format, coverage, PickCheck	automated	cargo test --workspace --locked --all-targets, cargo clippy ... -D warnings, cargo fmt --check, cargo run -- check crates, cargo llvm-cov --fail-under-lines 94 (94.79%), pickcheck check --base origin/main --fail-on new,worsened,unmatched (0 violations), on e8d0f01	pass	CI on this PR

QA: a CLI pass ran release builds from main (b7b327e) and this branch over a scratch Git repo in 14 scenarios, with a user config supplied through XDG_CONFIG_HOME, plus --changed and --base HEAD --format json with an unknown language. The nine language-entry cases differ from main only by naming the file and, for unknown names, reporting the name first. The evidence stays local and outside the repo.

docs/releases/UNRELEASED.md gains a Changes line for #43.

Review: independent xhigh reviews by Astra and Fable at e8d0f01 found no blocking issues, and Astra found nothing. Fable noted that the release line covers both the file path and the new check order. It stays one line because the order change comes from the same move. Fable also noted that value type errors still name no file, which is outside this issue.

Follow-up found during review: #46 (config value type errors do not name the file or key).