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/.gitignore

@@ -0,0 +1,3 @@
+# Generated by `relspec job run` in this example project.
+/build/
+/.relspec/

examples/jobs/migrations/core/1_001_create_users.sql

@@ -0,0 +1,4 @@
+CREATE TABLE users (
+    id SERIAL PRIMARY KEY,
+    email VARCHAR NOT NULL UNIQUE
+);

examples/jobs/migrations/tenant/1_002_create_posts.sql

@@ -0,0 +1,5 @@
+CREATE TABLE posts (
+    id SERIAL PRIMARY KEY,
+    user_id INT NOT NULL REFERENCES users(id),
+    title VARCHAR NOT NULL
+);

examples/jobs/migrations/tenant/2_001_add_posts_user_idx.sql

@@ -0,0 +1 @@
+CREATE INDEX posts_user_id_idx ON posts(user_id);

examples/jobs/relspec.yml

@@ -0,0 +1,45 @@
+# Example RelSpec job file. See docs/JOB_FILES.md for the full reference.
+#
+#   cd examples/jobs
+#   relspec job list
+#   relspec job run build-schema --plan
+#   relspec job run build-schema
+version: 1
+jobs:
+  build-schema:
+    command: convert
+    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
+
+  build-json:
+    command: convert
+    description: Also emit a JSON schema once build-schema succeeds
+    depends_on: [build-schema]
+    inputs:
+      - path: schema/core.dbml
+        format: dbml
+      - path: schema/tenant.dbml
+        format: dbml
+    output:
+      format: json
+      path: build/schema.json
+      overwrite: true
+
+  migration-order:
+    command: scripts-list
+    description: Show the combined execution order across script directories
+    script_dirs:
+      - migrations/core
+      - migrations/tenant
+    logfile: .relspec/log/migration-order.log

examples/jobs/schema/core.dbml

@@ -0,0 +1,5 @@
+Table users {
+  id int [pk, increment]
+  email varchar [not null, unique]
+  created_at timestamp
+}

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
+}