feat(cli): emit JSON envelope on init --agent --json reuse paths

PRD's documented form is `mem0 init --agent --json`, but rules 1/2
(env/config reuse) called printSuccess which is silenced under
agent mode — so the command exited 0 with no output.

Two related fixes:

1. Add `--json` as a subcommand-level option on init (Node only;
   Python's argv preprocessor already handles this). Lets the
   PRD-style invocation `mem0 init --agent --json` parse without
   "unknown option" error.

2. When agent mode is set AND a rule 1/2 reuse fires, emit the
   Dev Spec C4 envelope:
     {
       "status": "success", "command": "init",
       "data": {
         "api_key_saved": false,
         "api_key_source": "env" | "config",
         "agent_mode": false,
         "message": "Existing Mem0 API key found and reused..."
       }
     }
   Identical shape between Python and Node (parity).

Verified live: `init --agent --json` against prod with a valid
MEM0_API_KEY env returns the envelope above (no bootstrap call).

cli/python/src/mem0_cli/commands/init_cmd.py

@@ -258,23 +258,40 @@ def run_init(
     # is no valid key to reuse — in that case overwriting is correct.
     _agent_ctx = agent or _global_agent_mode() or (detect_agent_caller() is not None)
     if not api_key and not email and _agent_ctx:
+        from mem0_cli.output import format_json_envelope
+        from mem0_cli.state import is_agent_mode as _is_json_mode
+
+        def _emit_reuse(source: str) -> None:
+            if _is_json_mode():
+                format_json_envelope(
+                    console,
+                    command="init",
+                    data={
+                        "api_key_saved": False,
+                        "api_key_source": source,
+                        "agent_mode": False,
+                        "message": "Existing Mem0 API key found and reused. No Agent Mode key was created.",
+                    },
+                )
+            else:
+                msg = (
+                    "Existing MEM0_API_KEY is valid; reusing it. No new Agent Mode key was minted."
+                    if source == "env"
+                    else "Existing API key in config is valid; reusing it. No new Agent Mode key was minted."
+                )
+                print_success(console, msg)
+
         # Rule 1: env MEM0_API_KEY valid → reuse, no new key.
         _env_key = (os.environ.get("MEM0_API_KEY") or "").strip()
         if _env_key and _ping_key(_env_key, base_url):
-            print_success(
-                console,
-                "Existing MEM0_API_KEY is valid; reusing it. No new Agent Mode key was minted.",
-            )
+            _emit_reuse("env")
             _fire_init("existing_key")
             return
         # Rule 2: existing config api_key valid → reuse.
         if CONFIG_FILE.exists():
             _existing = load_config()
             if _existing.platform.api_key and _ping_key(_existing.platform.api_key, base_url):
-                print_success(
-                    console,
-                    "Existing API key in config is valid; reusing it. No new Agent Mode key was minted.",
-                )
+                _emit_reuse("config")
                 _fire_init("existing_key")
                 return
         # Rule 3: mint a fresh shadow (no valid key to reuse).

cli/python/src/mem0_cli/plugin_sync.py

@@ -26,6 +26,7 @@ Out of scope (deliberately not touched):
 
 from __future__ import annotations
 
+import contextlib
 import json
 import os
 import re
@@ -77,7 +78,10 @@ def _update_claude_settings(path: Path, api_key: str) -> bool:
 # Match `export MEM0_API_KEY="..."` (or single quotes, or no quotes).
 # Use [ \t]* (not \s*) for trailing whitespace so a trailing newline at
 # end-of-file is preserved when MEM0_API_KEY is the last line.
-_RC_LINE = re.compile(r'^([ \t]*export[ \t]+MEM0_API_KEY[ \t]*=[ \t]*)(["\']?)([^"\'\n]*)(["\']?)[ \t]*$', re.MULTILINE)
+_RC_LINE = re.compile(
+    r'^([ \t]*export[ \t]+MEM0_API_KEY[ \t]*=[ \t]*)(["\']?)([^"\'\n]*)(["\']?)[ \t]*$',
+    re.MULTILINE,
+)
 
 
 def _update_shell_rc(path: Path, api_key: str) -> bool:
@@ -110,8 +114,6 @@ def _atomic_write_text(path: Path, content: str) -> None:
             os.chmod(tmp_path, path.stat().st_mode & 0o777)
         os.replace(tmp_path, path)
     except Exception:
-        try:
+        with contextlib.suppress(OSError):
             os.unlink(tmp_path)
-        except OSError:
-            pass
         raise