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

docs/mcp-server-configuration.md

@@ -201,5 +201,6 @@ The CLI and TUI do not perform catalog CRUD yet — they are read consumers —
 
 ## Related
 
+- [Migrating from v1 to v2](./v1-to-v2-migration.md) — why `--config` means something narrower than it did in v1, and what `--catalog` replaced.
 - [Launcher and config consolidation](./launcher-config-consolidation-plan.md) — how the launcher and the shared config processor fit together.
 - [Reviewing an MCP App](./mcp-app-review.md) — the CLI-first App review recipe.