Fall back to $PGSERVICE when no connection string is set

[bluzername]

Closes #25.

pgx's ParseConfig already knows how to read a connection service
file - it
work fine if you give it service=myname. But pgbot return "no
connection string" and stop, before pgx even get a chance, when the only
thing set is $PGSERVICE.

I test this local: with PGSERVICEFILE and PGSERVICE set and nothing
else, pgxpool.ParseConfig("service=myname") resolve the host/port/user
from the service file correctly. So the fix is small - just add
$PGSERVICE as one more fallback in the same firstNonEmpty(...) chain
every command already use, after $DATABASE_URL / $PGBOT_DATABASE_URL,
same order the issue want.

What change:

• pgServiceFallback() in helpers.go turn a bare $PGSERVICE into
  "service=<name>", which pgx already understand.
• All 18 call sites (inspect, queries, vacuum, mcp, and the rest)
  now pass it as the last fallback.
• Test for the new helper, plus a fix to TestDsnFromArgs so it also
  clear $PGSERVICE in its "no connection string" case (else that test
  become flaky on a machine that has a service file configured).
• README: mention $PGSERVICE next to $DATABASE_URL in the 3 places
  that already document the fallback order.

An explicit argument or $DATABASE_URL/$PGBOT_DATABASE_URL still win,
nothing change there. go test ./... pass, gofmt/go vet clean.

[alexshapalov]

Code review

No issues found. Checked for bugs, all 18 call sites (none left without the fallback), the pgx side of the claim, and docs; this repo has no CLAUDE.md.

Verified that pgx v5.10 does what the description relies on: service=<name> is resolved through jackc/pgservicefile, honoring PGSERVICEFILE, including sslmode from the file. The PR's own test only covered building the string, so I pushed c15be83 to this branch (maintainer edits) with a test that writes a temp service file and asserts pgx applies its host, port, user, database, and sslmode, and that an unknown service name errors rather than falling through to localhost.

pgbot/cmd/pgbot/helpers.go

Lines 27 to 38 in cfc2789

	// pgServiceFallback lets a bare $PGSERVICE select a connection when neither an
	// argument nor $DATABASE_URL/$PGBOT_DATABASE_URL is set. pgx's ParseConfig
	// already reads a connection service file (PGSERVICEFILE, or the libpq
	// default path) once it gets a "service=..." string — this just builds that
	// string so users who manage connections through a service file don't have
	// to also pass one explicitly.
	func pgServiceFallback() string {
	if svc := os.Getenv("PGSERVICE"); svc != "" {
	return "service=" + svc
	}
	return ""
	}

One nit left alone: the "no connection string (pass one or set $DATABASE_URL)" message in each command doesn't mention $PGSERVICE; not worth touching 18 files for.

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