dx: make lint should detect editorconfig-checker under either binary name

[Chemaclass]

Context

Makefile hard-codes ec, but Homebrew installs the binary as editorconfig-checker. On such machines make lint fails (or is skipped) until a manual ec symlink is created. Hit during real use.

Scope

• Detect either name: LINTER_CHECKER := $(shell which ec 2>/dev/null || which editorconfig-checker 2>/dev/null) and invoke $(LINTER_CHECKER) bare (it auto-discovers .editorconfig).
• Keep the "not installed" guard message.

Boundary

Files: Makefile only. Quick, near-free.

Acceptance criteria

• make lint works whether the binary is ec or editorconfig-checker
• Still errors clearly when neither is installed
• CI linter job unaffected

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Constraints (apply to all bashdep work)

• Bash 3.2+ compatible — no declare -A, ${var,,}/${var^^}, mapfile, negative array index, &>>.
• Zero runtime deps beyond curl/wget/awk/mktemp/mkdir/rm/cp/mv/printf/cat/grep/sort/tr/dirname/basename + shell builtins.
• TDD: write the failing test first (tests/unit/*_test.sh), then implement; keep make test green for the right reason. Mock curl/wget — no network in tests.
• Quality gate: make test && make sa && make lint must all pass.
• Update docs/api.md / docs/behavior.md when public API or semantics change; add a CHANGELOG.md ## [Unreleased] entry for any user-visible change.
• Conventional commits (ref: for refactors); never mention AI/automation. One PR per issue, assigned to Chemaclass.