docs(config): add .env.example the README runbook references

Worker F's README told users to 'cp .env.example .env' but a blanket .env.*
gitignore rule silently dropped it. Un-ignore .env.example (real .env stays
ignored) and generate the template from internal/config: every TAPIR_* var,
which command needs it, accurate defaults, op/port-forward notes for demo time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -9,4 +9,5 @@
 # Local env / editor
 .env
 .env.*
+!.env.example
 *.local