Refactor `mds lint` CLI per-file dispatch spine

[dean0x]

Location: crates/mds-cli/src/lint.rs:738,822

lint_one_file_accumulating and lint_one_file_human are ~80-line near-identical twins; the FixFileOutcome match repeats in run_lint_file (~555) and run_lint_stdin (~462). Need behavior-preserving process_one_file spine + emit sink refactor.

Deferred From: PR #171 (Closes #61) — mds lint release gate
Review: .devflow/docs/reviews/feat-mds-lint-61/2026-07-11_1046/resolution-summary.md

Reason for Deferral:
Regression risk on the JSON/human exit paths under the v0.4.0 release gate. CLI spine refactoring is best done post-release when testing has stabilized.

Acceptance Criteria:

• Extract common process_one_file(path, mode) -> FixFileOutcome helper
• Reduce duplication in FixFileOutcome match handling
• Maintain behavior on JSON/human output paths
• All 590+ Rust tests + cli_lint.rs suite pass

[dean0x]

Cycle-2 review of PR #171 folds two more items into this refactor

The cycle-2 review of PR #171 (feat/mds-lint-61) surfaced two additional items that belong to this CPX-1 CLI-spine consolidation rather than as standalone issues. Capturing them here so they're addressed together:

• I-12 — run_lint_file's FixFileOutcome match (crates/mds-cli/src/lint.rs:576–598) is the third copy of the CPX-1 duplication (alongside lint_one_file_accumulating / lint_one_file_human and the run_lint_stdin match already noted in the description). Consolidate all three into the process_one_file spine.
• I-16 — directory-mode JSON aggregation (accumulate_result_json) allocates and discards the outer {version, files, truncated} wrapper per file. A clean fix wants a new mds-core LintResult::to_canonical_file_entry() method that yields a single file entry directly — this pairs naturally with the CLI-spine refactor and avoids the throwaway wrapper allocation.

Review: .devflow/docs/reviews/feat-mds-lint-61/2026-07-12_0020/resolution-summary.md

Suggested addition to the acceptance criteria:

• run_lint_file's FixFileOutcome match folds into the shared spine (no third copy)
• mds-core exposes LintResult::to_canonical_file_entry(); directory-mode JSON aggregation uses it instead of building+discarding the outer wrapper per file