docs: add an environment-variable reference (#2365)

Every variable that changes runtime behavior now lives on one page,
docs/environment-variables.md: which client reads it, its default, and
its effect. It covers three variables that were undocumented
(MCP_LOG_FILE, MCP_INSPECTOR_LOG_DIR, LOG_LEVEL), the secret-store
variables previously only in the Docker guide, and the Node TLS
variables that answer #1936 (a self-signed https://localhost server).

The README docs table, the migration guide, docker.md and the four
client READMEs link to it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TRJ64MnDTg1TpzCTEV9Xva
Signed-off-by: cliffhall <cliff@futurescale.com>

README.md

@@ -76,6 +76,7 @@ Each client has its own README with client-specific detail:
 | [Publishing](./docs/publishing.md) | What ships in the tarball, the packaging invariants, and `pack:verify` |
 | [Docker](./docs/docker.md) | Running the container image — ports, volumes, and where secrets go |
 | [Migrating from v1 to v2](./docs/v1-to-v2-migration.md) | CLI flag mapping, `--config` vs. `--catalog`, the Node engine bump, env-var renames |
+| [Environment variables](./docs/environment-variables.md) | Every variable that changes runtime behavior — auth, ports, storage, the secret store, logging, proxies — plus the Node TLS variables for a self-signed server |
 | [MCP server configuration](./docs/mcp-server-configuration.md) | Which server(s) the Inspector connects to, and the config file format |
 | [Reviewing an MCP App](./docs/mcp-app-review.md) | The CLI-first → one-shot-web recipe for automated App-tool review |
 | [Smoke-testing an MCP server](./docs/cli-smoke-testing.md) | The connect → list → call → assert workflow for a shell or CI job: `--format json` + `jq`, the exit-code map, and keeping OAuth non-interactive |