Tighten dev-config ergonomics

Apply the review feedback on the dev-config target:

- Treat cfg/dev.env as a prerequisite via $(wildcard ...) so edits
  to it re-trigger cfg/dev.yaml without the dev having to delete
  the output first; update the help string accordingly.
- Drop the @ silence prefix on the recipe body so failures are
  debuggable; the values are all known dev placeholders, no leak.
- Call out in cfg/dev.env.example that the file is sourced as a
  POSIX shell snippet (not Docker-compose .env semantics), and
  list the previously-missing overrides: observability addrs,
  PG_DEBUG, SMTP auth/TLS, AUTH_COOKIE_DURATION, and the
  per-worker LLM knobs (probo-agent, evidence-describer).

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

GNUmakefile

@@ -155,15 +155,15 @@ CFG_DEV_OAUTH2_KEY = cfg/.dev-oauth2-signing-key.pem
 CFG_DEV_ENV       = cfg/dev.env
 
 .PHONY: dev-config
-dev-config: cfg/dev.yaml ## Generate cfg/dev.yaml via probod-bootstrap (rerun after deleting the file)
+dev-config: cfg/dev.yaml ## Generate cfg/dev.yaml via probod-bootstrap (picks up edits to cfg/dev.env)
 
 $(CFG_DEV_OAUTH2_KEY):
 	@$(MKDIR) $(@D)
 	$(OPENSSL) genrsa -out $@ 2048
 
-cfg/dev.yaml: bin/probod-bootstrap $(CFG_DEV_OAUTH2_KEY)
+cfg/dev.yaml: bin/probod-bootstrap $(CFG_DEV_OAUTH2_KEY) $(wildcard $(CFG_DEV_ENV))
 	@$(MKDIR) $(@D)
-	@set -a; \
+	set -a; \
 	PROBOD_ENCRYPTION_KEY="thisisnotasecretAAAAAAAAAAAAAAAAAAAAAAAAAAA="; \
 	AUTH_COOKIE_SECRET="this-is-a-secure-secret-for-cookie-signing-at-least-32-bytes"; \
 	AUTH_PASSWORD_PEPPER="this-is-a-secure-pepper-for-password-hashing-at-least-32-bytes"; \