docs(spec): add agents plan show command with checkpoint listing, child plan status, and attempt count

Adds the `agents plan show <PLAN_ID>` CLI command specification.
This command was referenced in v3.3.0 milestone deliverables #7 and #10
but was never defined as a CLI command in the spec.

Deliverable #7: `plan show` lists checkpoints; each has a ULID and timestamp
Deliverable #10: `plan show` displays `Attempt: N` where N > 1 after correction

Closes proposal #6203 (UAT: agents plan show command missing from CLI).

Spec sections added:
- §CLI Commands — agents plan show: synopsis, arguments, Rich/Plain/JSON/YAML examples
- Documents checkpoint listing with ULID and timestamp
- Documents child plan status listing
- Documents attempt count display (Attempt: N after correction)
- Documents --format json machine-readable output

docs/specification.md

@@ -14172,6 +14172,140 @@ Status of a plan that encountered an error:
       - "Plan errored — use `agents plan prompt` to resume or `agents plan cancel` to abort"
     ```
 
+##### agents plan show
+
+<div class="highlight"><pre><code><span style="color: cyan; font-weight: 600;">agents</span> plan show <span style="color: #66cc66;">&lt;PLAN_ID&gt;</span></code></pre></div>
+
+**Purpose**
+Show detailed information for a plan, including checkpoint list, child plan statuses, and correction attempt count.
+
+**Arguments**
+
+- `<PLAN_ID>`: Plan ID (required).
+
+**Examples**
+
+=== "Rich"
+
+    <div class="highlight"><pre><code>
+    <span style="color: #66cc66; font-weight: 600;">$</span> <span style="color: cyan; font-weight: 600;">agents</span> plan show 01HXM8C2ZK4Q7C2B3F2R4VYV6J
+
+    ╭─ Plan Detail ────────────────────╮
+    │ <span style="color: cyan; font-weight: 600;">Plan:</span> 01HXM8C2ZK4Q7C2B3F2R4VYV6J │
+    │ <span style="color: yellow; font-weight: 600;">Phase:</span> applied                   │
+    │ <span style="color: magenta; font-weight: 600;">State:</span> applied                   │
+    │ <span style="color: #5599ff; font-weight: 600;">Action:</span> local/code-coverage      │
+    │ <span style="color: #66cc66; font-weight: 600;">Project:</span> local/api-service       │
+    │ <span style="color: #5599ff; font-weight: 600;">Automation:</span> review               │
+    │ <span style="color: #5599ff; font-weight: 600;">Attempt:</span> 2                       │
+    ╰──────────────────────────────────╯
+
+    ╭─ Checkpoints ─────────────────────────────────────────────────────╮
+    │ <span style="color: cyan; font-weight: 600;">ID</span>                          <span style="color: yellow; font-weight: 600;">Timestamp</span>              <span style="color: #66cc66; font-weight: 600;">Phase</span>     │
+    │ 01HXM8C2ZK4Q7C2B3F2R4VYV6K  2026-02-08T12:58:01Z   execute   │
+    │ 01HXM8C2ZK4Q7C2B3F2R4VYV6L  2026-02-08T13:01:45Z   execute   │
+    ╰───────────────────────────────────────────────────────────────────╯
+
+    ╭─ Child Plans ─────────────────────────────────────────────────────╮
+    │ <span style="color: cyan; font-weight: 600;">Plan ID</span>                      <span style="color: yellow; font-weight: 600;">State</span>      <span style="color: #66cc66; font-weight: 600;">Phase</span>     │
+    │ (none)                                                            │
+    ╰───────────────────────────────────────────────────────────────────╯
+
+    <span style="color: #66cc66; font-weight: 600;">✓ OK</span> Plan detail shown
+    </code></pre></div>
+
+=== "Plain"
+
+    ```
+    $ agents plan show 01HXM8C2ZK4Q7C2B3F2R4VYV6J
+
+    Plan Detail
+    -----------
+    Plan:       01HXM8C2ZK4Q7C2B3F2R4VYV6J
+    Phase:      applied
+    State:      applied
+    Action:     local/code-coverage
+    Project:    local/api-service
+    Automation: review
+    Attempt:    2
+
+    Checkpoints
+    -----------
+    ID                           Timestamp              Phase
+    01HXM8C2ZK4Q7C2B3F2R4VYV6K  2026-02-08T12:58:01Z   execute
+    01HXM8C2ZK4Q7C2B3F2R4VYV6L  2026-02-08T13:01:45Z   execute
+
+    Child Plans
+    -----------
+    (none)
+
+    [OK] Plan detail shown
+    ```
+
+=== "JSON"
+
+    ```json
+    {
+      "ok": true,
+      "command": "plan show",
+      "data": {
+        "plan_id": "01HXM8C2ZK4Q7C2B3F2R4VYV6J",
+        "phase": "applied",
+        "state": "applied",
+        "action": "local/code-coverage",
+        "project": "local/api-service",
+        "automation_profile": "review",
+        "attempt": 2,
+        "checkpoints": [
+          {
+            "id": "01HXM8C2ZK4Q7C2B3F2R4VYV6K",
+            "timestamp": "2026-02-08T12:58:01Z",
+            "phase": "execute"
+          },
+          {
+            "id": "01HXM8C2ZK4Q7C2B3F2R4VYV6L",
+            "timestamp": "2026-02-08T13:01:45Z",
+            "phase": "execute"
+          }
+        ],
+        "child_plans": []
+      },
+      "messages": ["Plan detail shown"]
+    }
+    ```
+
+=== "YAML"
+
+    ```yaml
+    ok: true
+    command: plan show
+    data:
+      plan_id: "01HXM8C2ZK4Q7C2B3F2R4VYV6J"
+      phase: applied
+      state: applied
+      action: local/code-coverage
+      project: local/api-service
+      automation_profile: review
+      attempt: 2
+      checkpoints:
+        - id: "01HXM8C2ZK4Q7C2B3F2R4VYV6K"
+          timestamp: "2026-02-08T12:58:01Z"
+          phase: execute
+        - id: "01HXM8C2ZK4Q7C2B3F2R4VYV6L"
+          timestamp: "2026-02-08T13:01:45Z"
+          phase: execute
+      child_plans: []
+    messages:
+      - "Plan detail shown"
+    ```
+
+**Notes**
+
+- `Attempt` is incremented each time `agents plan correct` is applied. After the first correction, `Attempt: 2` is displayed.
+- `Checkpoints` lists all checkpoints created during the Execute phase, each with a ULID identifier and ISO 8601 timestamp.
+- `Child Plans` lists all subplans spawned by this plan, with their current state and phase.
+- Downstream automation can parse `data` programmatically via `agents --format json plan show`.
+
 ##### agents plan cancel
 
 <div class="highlight"><pre><code><span style="color: cyan; font-weight: 600;">agents</span> plan cancel [(<span style="color: magenta;"><span style="color: cyan;">--reason</span>|<span style="color: yellow;">-r</span></span>) <span style="color: #66cc66;">&lt;REASON&gt;</span>] <span style="color: #66cc66;">&lt;PLAN_ID&gt;</span></code></pre></div>