feat(lint): add mise-usage-examples rule — enforce #USAGE example for public tasks (#43)

[olavostauros]

Summary

Adds a new lint rule mise-usage-examples that flags public .mise/tasks/* files with #USAGE arg/flag directives but no #USAGE example directives. Generalizes the check already done by bats-test-task (which only checks .mise/tasks/test) to all tasks.

Fixes #43.

Detection

• Scans $target/.mise/tasks/* files
• Skips tasks with #MISE hide=true
• Skips tasks with no #USAGE arg / #USAGE flag declarations
• Requires at least one #USAGE example directive when the task has a user-facing argument or flag surface
• Supports all standard ignore mechanisms (repo-level mise.toml, inline # codebase:ignore, rule-specific)

Self-hosting

The codebase repo itself had 12 public tasks with args/flags but no examples — all have been fixed in this PR with appropriate #USAGE example directives.

Fixtures (9 scenarios)

Fixture	Expectation
clean	tasks with examples, passes
dirty	tasks missing examples, flagged
hidden	#MISE hide=true, skipped
boolean-only	boolean flag without example, flagged
no-args	no USAGE arg/flag, skipped
ignored-inline	inline # codebase:ignore, passes
ignored-repo	repo-level ignore, SKIP
no-tasks	no .mise/tasks/ dir, OK
empty	empty .mise/tasks/ dir, OK
mixed	one clean + one dirty task, only dirty flagged

Files changed

File	Change
.mise/tasks/lint/mise-usage-examples	New lint rule (~90 lines)
test/lint/mise-usage-examples/mise-usage-examples.bats	12 BATS tests
test/lint/mise-usage-examples/fixtures/	10 fixture directories
.mise/tasks/pre-commit	Added 2 #USAGE example directives
.mise/tasks/scan	Added 1 #USAGE example directive
.mise/tasks/lint/_default	Added 1 #USAGE example directive
.mise/tasks/lint/bats-test-helper	Added 1 #USAGE example directive
.mise/tasks/lint/bats-test-task	Added 1 #USAGE example directive
.mise/tasks/lint/caller-pwd-contract	Added 1 #USAGE example directive
.mise/tasks/lint/github-actions	Added 2 #USAGE example directives
.mise/tasks/lint/gum-table	Added 1 #USAGE example directive
.mise/tasks/lint/mcr-scope	Added 1 #USAGE example directive
.mise/tasks/lint/mise-settings	Added 2 #USAGE example directives
.mise/tasks/lint/shellcheck	Added 1 #USAGE example directive
.mise/tasks/migrate/task-pattern	Added 2 #USAGE example directives
AGENTS.md	Added lint rules table

Validation

• mise run test — 230/230 tests pass (12 new + 218 existing, zero regressions)
• codebase lint:mise-usage-examples . — self-hosting passes clean
• git diff --check — no whitespace errors