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>

README.md

@@ -106,6 +106,49 @@ Modes: `database` (default) · `schema` · `table` · `script`
 
 Template functions: string utils (`toCamelCase`, `toSnakeCase`, `pluralize`, …), type converters (`sqlToGo`, `sqlToTypeScript`, …), filters, loop helpers, safe access.
 
+### `job` — Declarative job files
+
+Run named jobs from a `relspec.yml` manifest instead of repeating long command lines.
+
+```bash
+# List jobs discovered in ./relspec.yml and ./relspec.<name>.yml (deterministic)
+relspec job list
+
+# Validate and print the plan without running anything
+relspec job run build-schema --plan
+
+# Run a job (and its declared dependencies)
+relspec job run build-schema
+```
+
+```yaml
+# relspec.yml
+version: 1
+jobs:
+  build-schema:
+    command: convert            # closed allow-list: convert | merge | scripts-list
+    description: Merge the DBML sources and emit PostgreSQL DDL
+    inputs:
+      - path: schema/core.dbml
+        format: dbml
+      - path: schema/tenant.dbml
+        format: dbml
+    output:
+      format: pgsql
+      path: build/schema.sql
+      overwrite: true
+    options:
+      flatten_schema: false
+    logfile: .relspec/log/build-schema.log
+```
+
+The job system is **not** a shell: `command` is a fixed enum, every path is
+resolved relative to the job file and may not escape it, and remote database
+credentials are referenced by environment-variable name (`conn_env:`) and
+redacted from logs. The whole plan — unknown commands/formats, duplicate job
+names, missing inputs, path traversal, dependency cycles — is validated before
+any job runs. See [docs/JOB_FILES.md](docs/JOB_FILES.md).
+
 ### `edit` — Interactive TUI editor
 
 ```bash