Support per-user identity via request headers

Add multi-user auth: _config reads X-Freshservice-Domain/X-Freshservice-Key
from the per-request headers (via the SDK request_ctx, set inside the tool's
task) and prefers them over the container env. One shared server can now act as
each caller's own Freshservice identity — actions attributed to them, their
permissions, their rate limit — while the env vars remain a single-user
fallback. Verified end-to-end that concurrent clients don't cross-wire and that
a client with no creds errors instead of falling back silently.

Docs/.env.example updated with the multi-user client config and the guidance to
omit env FS creds on a shared deployment.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mcp-server/.env.example

@@ -1,4 +1,11 @@
-FRESHSERVICE_DOMAIN=pesco
-FRESHSERVICE_API_KEY=your_freshservice_api_key
-# Shared bearer token the Claude Desktop client must send. Generate: openssl rand -hex 32
+# Shared bearer token every client must send to reach the server (the "gate").
+# Generate: openssl rand -hex 32
 MCP_AUTH_TOKEN=replace_with_a_long_random_token
+
+# --- Single-user only -------------------------------------------------------
+# Uncomment to bake ONE identity into the server. Every client then acts as
+# this Freshservice user. For a shared/multi-user server, LEAVE THESE OUT and
+# have each client send its own X-Freshservice-Domain / X-Freshservice-Key
+# headers instead (see README → "Multiple users").
+# FRESHSERVICE_DOMAIN=pesco
+# FRESHSERVICE_API_KEY=your_freshservice_api_key