…tory (#38)

Move remaining 4 entity domains (mcp, plugin, file, memory) to
src/features/ completing the co-located feature directory pattern
for all 7 domains (hook, skill, subagent already migrated).

Each domain now follows the standard structure:
  src/features/{domain}/
    config.ts, service.ts, server.ts, queries.ts,
    utils.ts/constants.ts, components/, *.test.ts

Removed scattered files from services/, server/, hooks/, lib/,
components/, config/entities/. Updated ~50 import references
across the codebase. Added 16 integration tests for migration
integrity and restored 19 getPluginMcpServers unit tests.

Constraint: shared files (entity-inspector, entities/index, BoardLayout) updated in separate phase to avoid parallel agent conflicts
Rejected: separate worktrees per domain | shared file merge conflicts outweigh isolation benefit
Confidence: high
Scope-risk: broad
Not-tested: runtime UI rendering (only static imports and unit tests verified)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>