Guard lint.mk's submodule include so it doesn't break tarball builds

[jnasbyupgrade]

Summary

From `~/test-fixes.md` item #12 (found in `cat_tools`): an unconditional `include` of a submodule-vendored `.mk` file with a self-init rule breaks every `make` invocation, not just the feature it wires up, when building from a source tarball (`git archive`, PGXN's own `make dist` output) rather than a real git checkout.

test_factory has this exact pattern: `Makefile` does `include lint.mk`, and `lint.mk` does `include .vendor/linter/lint.mk` with a self-init rule (`git submodule update --init`). Make tries to satisfy every `include` before doing anything else, for any target requested. A real distribution tarball has no `.git` and no submodule content, so `git submodule update` fails immediately (`fatal: not a git repository`), and that failure aborted the entire build -- `make`, `make install`, everything, not just `make lint`.

Confirmed, not assumed

Built a real tarball (`git archive` of current `master`) into a clean directory with no `.git` at all:

• Before this fix: plain `make` failed immediately, exit code 2, `fatal: not a git repository`.
• After this fix: plain `make` succeeds (exit 0), `make install` succeeds (exit 0), and `make lint` there now fails with Make's own clean `No rule to make target 'lint'` -- a specific, targeted failure only when lint is actually requested, instead of aborting every target.

Also confirmed the fix doesn't regress the normal case: from a real git checkout, both `make lint` (submodule auto-inits exactly as before) and a plain `make` still succeed.

Fix

Guard the self-init rule and the submodule `include` behind `ifneq ($(wildcard .git),)`. Outside a real git checkout, lint support is simply unavailable -- nothing else in the build needs it.

Test plan

• Real git checkout: `make lint` still works
• Real git checkout: plain `make` still works
• Tarball (`git archive`, no `.git`): plain `make` now succeeds (previously failed)
• Tarball: `make install` succeeds
• Tarball: `make lint` fails gracefully (`No rule to make target`) instead of aborting everything

🤖 Generated with Claude Code

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