Guard vendoring template's lint.mk include against tarball builds

[jnasbyupgrade]

Summary

• The "Consuming this repo" README template's lint.mk include was unconditional, so a git archive tarball build (no .git, no submodule content — what make dist/PGXN release archives use) would abort the whole Makefile parse trying to auto-init .vendor/linter, breaking make/make install entirely.
• Wraps the template's auto-init rule and include in ifneq ($(wildcard .git),), matching the fix applied downstream in cat_tools PR #80.
• This repo's README is the reference example other Postgres-Extensions repos copy, so the guard belongs here at the source rather than only being fixed per-consumer.

Test plan

• Confirm normal checkouts (.git present) still get make lint working as before.

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