docs: restore SECURITY.md, lost in the v2 tree swap (#1843)

`SECURITY.md` existed on the pre-swap `main` (ac3c1a12) and still exists on
`v1/main`, but the v2 tree never had one — so #1817 silently removed the
security policy from the default branch.

That is the wrong file to be missing right now: it is where private
vulnerability reporting is advertised, and it disappeared in the same window as
2.0.0 shipping and external pull requests being turned off (#1820). Losing the
PR path and the security-report path together would leave an outside reporter
with no documented route at all.

Restored from `v1/main` with the additions #1820 asks for:

- A supported-versions table covering all four package names — v2 supported, the
  v1 line security-fixes-only under the `v1-latest` dist-tag, anything below
  1.0.0 unsupported. The three sub-packages are named explicitly since they
  exist only on v1 and are deprecated.
- A note that the repository does not accept outside pull requests, but that
  this explicitly does NOT apply to security reports, which must go through the
  advisory process rather than any public channel. Without that, the new
  issues-only policy reads as "no way to reach us".
- "Whether it affects v2, v1, or both" added to the report checklist, now that
  two lines are live.

Verified private vulnerability reporting is enabled on the repo, so the advisory
link is a working route rather than an aspiration.


Claude-Session: https://claude.ai/code/session_01Txmv2qqv3yeKgRzoqXytzD

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

SECURITY.md

@@ -0,0 +1,53 @@
+# Security Policy
+
+Thank you for helping keep the Model Context Protocol and its ecosystem secure.
+
+## Supported Versions
+
+| Version | Package                                  | Support                                                 |
+| ------- | ---------------------------------------- | ------------------------------------------------------- |
+| **2.x** | `@modelcontextprotocol/inspector`        | ✅ Actively supported                                   |
+| 1.x     | `@modelcontextprotocol/inspector`        | ⚠️ **Security fixes only**, published under `v1-latest` |
+| 1.x     | `@modelcontextprotocol/inspector-client` | ⚠️ Security fixes only                                  |
+| 1.x     | `@modelcontextprotocol/inspector-server` | ⚠️ Security fixes only                                  |
+| 1.x     | `@modelcontextprotocol/inspector-cli`    | ⚠️ Security fixes only                                  |
+| < 1.0.0 | any                                      | ❌ Unsupported                                          |
+
+v2 ships as a **single** package — `@modelcontextprotocol/inspector`. The three
+`inspector-client` / `inspector-server` / `inspector-cli` sub-packages exist only
+on the v1 line and are deprecated.
+
+v1 development happens on the `v1/main` branch and is limited to security fixes.
+Those releases are published under the **`v1-latest`** dist-tag so they never
+displace the current v2 release:
+
+```bash
+npm i @modelcontextprotocol/inspector@v1-latest
+```
+
+Everything else — features, bug fixes, improvements — lands in v2 only.
+
+## Reporting Security Issues
+
+If you discover a security vulnerability in this repository, please report it through
+the [GitHub Security Advisory process](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing-information-about-vulnerabilities/privately-reporting-a-security-vulnerability)
+for this repository. Private vulnerability reporting is enabled, so this is the
+fastest route to a maintainer.
+
+Please **do not** report security vulnerabilities through public GitHub issues, discussions,
+or pull requests.
+
+Note that this repository does not accept pull requests from outside contributors
+(see [CONTRIBUTORS.md](./CONTRIBUTORS.md)) — **this does not apply to security
+reports**, which should always go through the advisory process above rather than
+any public channel.
+
+## What to Include
+
+To help us triage and respond quickly, please include:
+
+- A description of the vulnerability
+- Steps to reproduce the issue
+- The potential impact
+- Whether it affects v2, v1, or both
+- Any suggested fixes (optional)