feat: add mise-run-quiet lint rule (#42)

[olavostauros]

Summary

Adds lint:mise-run-quiet rule that flags mise run calls without -q in clean-output contexts where task runner headers pollute captured output (implements issue #42).

Detection

File-scan with per-line context classification via discover_shell_files():

Context	Severity	Example
Command substitution	FAIL	$(mise run build) — header goes into captured output
eval	FAIL	eval "\$(mise run env)" — header pollutes eval'd string
Pipeline	FAIL	`mise run check

Safe contexts (not flagged): bare interactive calls, mise run -q, 2>/dev/null, grep/sed/awk patterns, comments, string literals.

Files added

File	Description
.mise/tasks/lint/mise-run-quiet	New lint rule (~227 lines)
test/lint/mise-run-quiet/mise-run-quiet.bats	17 BATS tests
test/lint/mise-run-quiet/fixtures/	9 fixture directories (clean, clean-already-quiet, dirty-command-sub, dirty-eval, dirty-pipe, dirty-mixed, ignored-file, ignored-inline, no-shell-files)

Validation

• ✅ 235/235 tests pass (17 new mise-run-quiet tests at indices 106–122, zero regressions)
• ✅ Self-hosting: mise run lint:mise-run-quiet . — only test fixtures flagged, codebase repo clean
• ✅ ShellCheck: Clean
• ✅ Whitespace: git diff --check — no errors

Closes #42