refactor: Move guard-policy parsing, AllowOnly factory, and DIFC sink state to proper packages

[Copilot]

Summary

Addresses the three most impactful semantic function clustering refactoring opportunities identified in the issue.

Changes

1. Guard-policy parsing moved to config package

• parseServerGuardPolicy and parsePolicyMap moved from internal/server/unified.go → internal/config/guard_policy.go, exported as ParseServerGuardPolicy / ParsePolicyMap
• These are domain parsing functions that understand the GuardPolicy structure and should live alongside the type definitions, not in the transport/server layer
• internal/server/unified.go now delegates to config.ParseServerGuardPolicy

2. BuildAllowOnlyPolicy factory moved to config package

• buildAllowOnlyPolicy moved from internal/cmd/flags_difc.go → internal/config/guard_policy.go, exported as BuildAllowOnlyPolicy
• Policy construction is domain logic and belongs next to the policy types, not in the CLI layer
• internal/cmd/root.go now delegates to config.BuildAllowOnlyPolicy

3. DIFC sink server ID state moved to difc package

• difcSinkServerIDs global state, SetDIFCSinkServerIDs, and isDIFCSinkServerID removed from internal/mcp/connection.go
• New file internal/difc/sink_server_ids.go exports SetSinkServerIDs and IsSinkServerID
• The transport layer (mcp) should not own security policy state — it now imports difc and delegates to difc.IsSinkServerID
• internal/cmd/root.go now calls difc.SetSinkServerIDs directly

Testing

All existing tests pass (make agent-finished). Call sites updated throughout.

Closes #[issue-number]

[Copilot]

Pull request overview

Refactors DIFC/guard-policy domain logic out of transport/CLI layers into the config and difc packages, reducing semantic clustering in server and mcp while keeping behavior/call-sites consistent.

Changes:

• Moved server guard-policy parsing helpers into internal/config (ParseServerGuardPolicy, ParsePolicyMap) and updated UnifiedServer + tests to call them.
• Moved AllowOnly policy factory into internal/config (BuildAllowOnlyPolicy) and updated CLI override resolution + tests accordingly.
• Moved DIFC sink server ID global state into internal/difc (SetSinkServerIDs, IsSinkServerID), updated mcp logging enrichment and CLI wiring.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/server/unified.go	Delegates server guard-policy parsing to config.ParseServerGuardPolicy; removes local parsing helpers.
internal/server/guard_policy_parsing_test.go	Updates tests to call config.ParseServerGuardPolicy.
internal/config/guard_policy.go	Adds exported parsing helpers and BuildAllowOnlyPolicy factory.
internal/cmd/root.go	Wires sink server IDs via difc.SetSinkServerIDs; builds AllowOnly policy via config.BuildAllowOnlyPolicy.
internal/cmd/flags_difc.go	Removes CLI-local buildAllowOnlyPolicy helper.
internal/cmd/flags_difc_test.go	Updates tests to call config.BuildAllowOnlyPolicy.
internal/mcp/connection.go	Delegates sink-server check to difc.IsSinkServerID for tag snapshot enrichment.
internal/mcp/connection_test.go	Updates sink-server ID tests to use difc API.
internal/mcp/sdk_method_dispatch_test.go	Updates tests to configure sink-server IDs via difc.SetSinkServerIDs.
internal/difc/sink_server_ids.go	Introduces concurrency-safe global sink server ID set and lookup helpers.

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