docs: add the v1 → v2 migration guide

Closes #1822

Adds docs/v1-to-v2-migration.md — the map for users arriving from v1:

- CLI flag mapping (every v1 flag survives; lists what v2 adds)
- `--config` vs `--catalog` semantics with before/after examples
- Node engine bump (>=22.7.5 → >=22.19.0)
- what no longer ships (the three sub-packages; v2 is one tarball)
- the three behavior changes most likely to bite: exit code 5 on a
  failing tool call, target-must-come-first under --cli, and the
  reversed `--` separator
- env-var mapping (MCP_PROXY_AUTH_TOKEN → MCP_INSPECTOR_API_TOKEN,
  MCP_PROXY_FULL_ADDRESS removed, SERVER_PORT repurposed), the v1 UI
  config settings, web query params, and Docker

Linked from the root README (banner + docs list), the CLI README, and
docs/mcp-server-configuration.md; AGENTS.md's docs/ tree entry updated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SW1p8E2uiyyLx4RKwrwSrt

clients/cli/README.md

@@ -12,6 +12,8 @@ npx @modelcontextprotocol/inspector --cli node build/index.js
 
 Supports tools, resources, and prompts (plus `--method servers/list` / `servers/show` for catalog entries without connecting).
 
+> Coming from the v1 CLI? See the [v1 → v2 migration guide](../../docs/v1-to-v2-migration.md) — every v1 flag still exists, but exit codes, argument ordering, and the `--` separator changed.
+
 ### Examples
 
 **Basic usage**