feat(P03): task execution engine with HCL specs, jobs, tasks, WaitDelay

Implements Phase 3 of v0.1 Foundation:
- internal/model/job.go: Job + Task models with status state machines
- internal/store/migrations/0002_jobs_tasks.sql: jobs + tasks tables with FK
- internal/store/job_task_repo.go: JobRepo + TaskRepo with CRUD and lifecycle updates
- internal/jobspec/spec.go: HCL parser using hashicorp/hcl/v2 hclsimple
- internal/jobspec/spec_test.go: 4 tests for parser
- internal/engine/executor.go: parallel task executor using os/exec with Go 1.25
  WaitDelay for clean process shutdown
- internal/cli/job.go: orca job {run,list,stop,logs} wired to executor
- testdata/hello.hcl, testdata/fail.hcl: smoke test fixtures

Verified: job run executes commands, captures stdout/stderr, persists state,
job stop transitions status, job logs displays captured output. All tests
pass with -race.

---ci---
project: orca
phase: 3
milestone: v0.1
status: execute
req_covered:
  - REQ-004
  - REQ-006
  - REQ-009
  - REQ-018
  - REQ-020
  - REQ-021
---/ci---

testdata/fail.hcl

@@ -0,0 +1,7 @@
+job "failing-job" {
+}
+
+task "fail" {
+  command = "/bin/sh"
+  args    = ["-c", "echo oops 1>&2; exit 1"]
+}

testdata/hello.hcl

@@ -0,0 +1,7 @@
+job "hello-orca" {
+}
+
+task "greet" {
+  command = "/bin/echo"
+  args    = ["hello", "from", "orca"]
+}