refactor(implement-issue): replace @{prompt} placeholder with {prompt} and remove deprecated --agent flag

Replace the `@{prompt}` placeholder with `{prompt}` across all skill docs and
agent-cli configs, remove the deprecated `--agent` flag from the invocation
syntax, delete the now-obsolete agent-cli test file, clarify that only one
triage label should be active at a time, and defer prompt-file cleanup to the
OS/child instead of removing it immediately after tmux launch.

common/engineering/setup-skills/agents-seed.md

@@ -9,7 +9,7 @@ agents:
   - name: pi
     binary: pi
     args: "@{prompt}"
-    description: "My primary agent harness. Accepts prompt file via @{prompt}."
+    description: "My primary agent harness. Accepts prompt file via {prompt}."
 
   - name: opencode
     binary: opencode
@@ -26,7 +26,7 @@ agents:
   - name: codex
     binary: codex
     args: "@{prompt}"
-    description: "OpenAI Codex. Accepts prompt file via @{prompt}."
+    description: "OpenAI Codex. Accepts prompt file via {prompt}."
 
   - name: claude
     binary: claude
@@ -41,6 +41,6 @@ agents:
 |-------|-------------|
 | `name` | Display name used in menus and `--agent` flag |
 | `binary` | Command name expected on PATH |
-| `args` | Static arguments appended after the binary. May include `@{prompt}` which is substituted at invocation time with the absolute path to the prompt file. Empty string means stdin piping (prompt is piped via `cat`). |
+| `args` | Static arguments appended after the binary. May include `{prompt}` which is substituted at invocation time with the absolute path to the prompt file. Empty string means stdin piping (prompt is piped via `cat`). |
 | `description` | Short human-readable description for the setup menu |
 | `note` | Optional additional context |