Generate cfg/dev.yaml via a Make target

Committing a fully-materialised cfg/dev.yaml hid the dev
configuration surface and blocked the OAuth2 signing-key inlining
change: the new config requires a per-dev private key that must
not be committed. Replace the checked-in file with a dev-config
Make target that shells out to probod-bootstrap with dev-safe
defaults and a stable RSA signing key stashed under
cfg/.dev-oauth2-signing-key.pem on first run.

The recipe sources cfg/dev.env when present so devs can override
any setting without editing the Makefile; cfg/dev.env.example
ships the full list of overridable knobs. cfg/dev.yaml,
cfg/dev.env, and the signing key are all gitignored.

Update README, CONTRIBUTING, and contrib/claude/config.md to
describe the new workflow.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

.gitignore

@@ -20,3 +20,5 @@ __generated__/
 pkg/server/api/*/v1/types/types.go
 pkg/server/api/*/v1/schema.graphql
 cfg/dev_local.yaml
+cfg/dev.yaml
+cfg/dev.env