refactor: semantic function clustering — absorb dockerutil, reduce duplication, move outlier free functions

[Copilot]

Summary

Addresses the actionable improvements identified in issue #1827 (semantic function clustering analysis).

Changes

1. Absorb internal/dockerutil into internal/mcp

The internal/dockerutil package contained a single function (ExpandEnvArgs) used exclusively by internal/mcp/connection.go. Moving it eliminates the single-function package:

• Created internal/mcp/dockerenv.go with ExpandEnvArgs
• Updated connection.go to use the local function (no more import of dockerutil)
• Consolidated TestExpandDockerEnvArgs in connection_test.go with the 4 additional test cases from the old dockerutil/env_test.go
• Deleted internal/dockerutil/

2. Reduce duplication in auth.TruncateSessionID

TruncateSessionID in internal/auth/header.go duplicated the truncation logic already in internal/strutil/truncate.go. It now delegates to strutil.Truncate(sessionID, 8), keeping only the "(none)" special-case for empty strings.

3. Move toDIFCTags/tagsToStrings from unified.go to internal/difc

These free functions only deal with difc.Tag types and have no dependency on server logic. Exported as difc.StringsToTags / difc.TagsToStrings in a new internal/difc/tags.go file with unit tests.

4. Move convertToCallToolResult/parseToolArguments from unified.go to internal/mcp

These MCP SDK utility functions are generic enough to belong in the mcp package alongside other protocol helpers. Exported as mcp.ConvertToCallToolResult / mcp.ParseToolArguments in internal/mcp/tool_result.go, using logger.New("mcp:tool_result") for debug logging per project conventions.

Testing

• All existing unit and integration tests pass (make agent-finished)
• New unit tests added for difc.StringsToTags / difc.TagsToStrings
• CodeQL: no alerts

Closes #1827

[Copilot]

Pull request overview

This PR refactors and “clusters” semantically related helper functions into the packages where they logically belong, removing a single-function package and reducing duplication across the codebase.

Changes:

• Moved Docker -e VAR environment expansion logic into internal/mcp and removed internal/dockerutil.
• Deduplicated auth.TruncateSessionID by delegating to strutil.Truncate.
• Moved DIFC tag conversion helpers out of UnifiedServer into internal/difc with new unit tests.
• Moved MCP tool result/argument parsing helpers out of UnifiedServer into internal/mcp, updating call sites and tests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/server/unified.go	Switched to new mcp/difc helper functions; removed local free functions.
internal/server/call_tool_result_test.go	Updated tests to call mcp.ConvertToCallToolResult.
internal/mcp/tool_result.go	New home for ConvertToCallToolResult and ParseToolArguments with debug logging.
internal/mcp/dockerenv.go	Absorbed Docker env expansion helper into mcp with updated logger namespace.
internal/mcp/connection.go	Uses local ExpandEnvArgs instead of dockerutil.
internal/mcp/connection_test.go	Consolidated/expanded Docker env arg tests; removed dockerutil import.
internal/difc/tags.go	New exported DIFC tag/string conversion helpers.
internal/difc/tags_test.go	Unit tests for StringsToTags / TagsToStrings.
internal/auth/header.go	TruncateSessionID now delegates to strutil.Truncate.
internal/dockerutil/env_test.go	Removed as part of deleting the internal/dockerutil package.

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