feat(ai-chat): per-MCP-server instructions in the agent system prompt (#180)

Admins can now give each EXTERNAL MCP server a free-text instruction ("how/
when to use this server's tools") that the agent receives in its SYSTEM
PROMPT next to the tool descriptions — porting the built-in SERVER_INSTRUCTIONS
idea to admin-configured servers. Trusted, admin-authored text (like a system
prompt); NON-secret, so unlike headersEnc it IS returned in views/forms.

- Migration: nullable `instructions text` on ai_mcp_servers (old rows = null =
  no guidance). Table type + repo insert/update (blank/whitespace -> null via
  blankToNull). DTO `@MaxLength(4000)`. Service threads it through
  McpServerView/toView.
- mcp-clients: `McpServerInstruction { serverName, toolPrefix, instructions }`
  threaded through the toolset/cache/lease. Guidance is built ONLY for a server
  that actually connected AND contributed >=1 callable tool (the allowlist may
  filter all of them out) AND has non-blank text — so a guide never appears for
  tools the agent cannot call. Cached with the toolset, so an edit is picked up
  next turn via the existing CRUD cache invalidation.
- System prompt: `buildMcpToolingBlock` renders an <mcp_tooling> block INSIDE
  the safety sandwich (after context, before the trailing SAFETY_FRAMEWORK) so
  it informs tool choice but cannot override the rules; each section is headed
  by the server's `prefix_*` namespace. Empty/blank -> block omitted. The
  caller (ai-chat.service) now builds the external toolset BEFORE the prompt and
  passes external.instructions; client-handle lifecycle (close-once) unchanged.
- Client: instructions field in types + a Textarea (autosize, maxLength 4000)
  in the MCP-server form with a namespace-prefix hint; i18n (en/ru).

Tests across every layer (prompt block placement + both SAFETY copies; view
blank->null; buildEntry includes guidance only for connected+>=1-tool+non-blank;
DTO MaxLength; repo + integration round-trip; service wiring). Delegated impl
reviewed (APPROVE); applied the import-type follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/public/locales/en-US/translation.json

@@ -710,6 +710,7 @@
   "Authorization header": "Authorization header",
   "Tool allowlist": "Tool allowlist",
   "Optional. Leave empty to allow all tools the server exposes.": "Optional. Leave empty to allow all tools the server exposes.",
+  "Optional guidance for the agent on how and when to use this server's tools. Injected into the system prompt. The server's tools are namespaced as \"<server name>_*\".": "Optional guidance for the agent on how and when to use this server's tools. Injected into the system prompt. The server's tools are namespaced as \"<server name>_*\".",
   "Test": "Test",
   "Available tools": "Available tools",
   "No tools available": "No tools available",