feat(job): declarative YAML job files for named relspec workflows

Add `relspec job list` and `relspec job run <name>` driven by YAML job
manifests (relspec.yml / relspec.<name>.yml), so multi-file merge and
conversion workflows can be expressed declaratively instead of as long
shell command lines.

v1 contract (see docs/JOB_FILES.md):
- `command` is a closed allow-list (convert, merge, scripts-list); no
  field accepts a shell string or executable path.
- Deterministic discovery: default file first, then named files sorted
  lexically; all files merged into one namespace; duplicate job names
  across files are a hard error.
- Every path resolves relative to the job file's directory; absolute,
  home-relative and directory-escaping paths are rejected at validation.
- Database credentials referenced by env-var name via `conn_env:`;
  connection strings are never stored and are redacted from logs/plan.
- Full validation (version, unknown fields, command/format, per-command
  input/output shape, path traversal, depends_on targets, dependency
  cycles) runs before anything is read, written or executed; per-job
  pre-flight then checks input existence, script dirs, env vars and the
  output overwrite policy for the whole plan.
- `depends_on` closure runs in deterministic topological order;
  `--no-deps` runs only the named job.
- `--dry-run` (alias `--plan`) prints the resolved plan and exits 0
  without touching inputs, outputs or databases.
- A failing job propagates the underlying non-zero exit status, logs
  FAILED (never OK), and writes no success marker.

pkg/jobs is side-effect free (discovery/parse/validate/plan only);
execution adapters live in cmd/relspec/job.go. Includes unit tests for
discovery, validation, planning and path safety, plus CLI tests for
end-to-end convert/merge, scripts-list across multiple directories,
dry-run, dependency chains, exit-code propagation and log redaction.

Deferred: live `scripts execute` from jobs, split/inspect/diff/templ
commands, job-to-job output wiring, log rotation/retention.

Refs #20

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

examples/jobs/schema/tenant.dbml

@@ -0,0 +1,6 @@
+Table posts {
+  id int [pk, increment]
+  user_id int [not null, ref: > users.id]
+  title varchar [not null]
+  body text
+}