feat(mcp): allow deployments to override the server instructions

[sgral]

Summary

GRAPHITI_MCP_INSTRUCTIONS is a module-level constant passed straight into FastMCP(...), so the only way to change what MCP clients are told on connect is to edit graphiti_mcp_server.py. For anyone running the server as a container that means maintaining a patched image purely to carry a few paragraphs of text.

That text is exactly where a deployment's own conventions belong — which group_ids exist, how to route facts between them, when to search before answering. The built-in default cannot know any of that, and it is the first thing every connecting client reads.

Change

A small resolve_server_instructions() helper, checked in order:

1. GRAPHITI_MCP_INSTRUCTIONS — the instructions themselves
2. GRAPHITI_MCP_INSTRUCTIONS_FILE — a path to read them from
3. the built-in default (unchanged)

mcp = FastMCP(
    'Graphiti Agent Memory',
    instructions=resolve_server_instructions(),
)

Why environment and not config.yaml: the FastMCP instance is constructed at import time, before the configuration file is loaded. FastMCP.instructions is a read-only property over self._mcp_server.instructions, so setting it later in initialize_server() would mean writing to a private attribute. Resolving before construction keeps this to public API only.

Why a file path as well as an inline variable: this field is normally multi-paragraph, which is awkward to carry in an environment variable and unpleasant to quote in compose files.

Unreadable file falls back rather than raising — it logs a warning and uses the default. Wrong instructions are a much smaller problem than a memory server that will not start.

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation/Tests

Objective

Let deployments describe their own memory conventions without forking the image. This came out of running the MCP server against FalkorDB with several active group_ids, where the routing rules an agent needs are entirely site-specific.

Testing

• Unit tests added — mcp_server/tests/test_server_instructions.py, five cases: default, inline override, file override, inline-beats-file precedence, and unreadable-file fallback.
• Integration tests added/updated
• Existing tests unaffected — the change is additive and the default path is byte-identical to before.

Note: I verified the resolution order by exercising the function's logic directly (all five cases pass) but could not run the repository's pytest suite in my environment. Worth a CI run.

Breaking Changes

• This PR contains breaking changes

With neither variable set, behaviour is exactly as before.

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated where necessary — both variables added to the environment table in mcp_server/README.md
• No secrets or sensitive information committed

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[sgral]

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