Clean up Epicea change history metadata

[Gabriel-Darbord]

Summary

• remove MCP extensions for legacy Monticello Epicea save/load events
• replace change-history selector probing with Epicea event double dispatch
• remove the Object/OmReference fallback extensions and obsolete MCPChangeHistoryValueExtractor
• tighten change-history entry initialization contracts and remove redundant trailing receiver returns
• document that methods without explicit returns answer self
• keep change-history listing, filtering, apply, and revert behavior unchanged

Epicea code-change events now answer a narrow MCP hook that fills MCPChangeHistoryEntryInfo. The entry DTO owns the output fields, while event subclasses choose the appropriate metadata path by dispatch.

Verification

• P13 direct MCP: MCPToolChangeHistoryTest and MCPToolChangeHistoryToolsTest passed (22 tests)
• P13 direct MCP: MCPToolContractsTest passed (101 tests)
• P13 direct MCP: MCPToolStructuredOutputTest passed (29 tests)
• P13 direct MCP: no remaining mcpChangeHistoryValueForSelector:, mcpChangeHistoryReferenceString, or MCPChangeHistoryValueExtractor references in MCP packages
• P13 disk scan: no ifNil: [ ^ self ] remains in change-history classes
• P14 direct MCP: baseline MCP loaded from the local Tonel checkout without warnings
• P14 direct MCP: MCPToolChangeHistoryTest and MCPToolChangeHistoryToolsTest passed (22 tests)
• P13 direct MCP: repository is clean after adopting HEAD
• git diff --check passed