feat(export): surface MCP server usage in JSON and CSV exports (#496)

[iamtoruk]

Problem

codeburn export never showed MCP usage even when sessions had MCP activity (#496). Two distinct gaps:

1. MCP usage wasn't parsed for recent Codex sessions (the mcp_tool_call_end event was unhandled). Fixed separately in fix(codex): attribute MCP calls emitted as event_msg/mcp_tool_call_end (#478) #513, which now populates session.mcpBreakdown.
2. The export schema never surfaced MCP at all (this PR). exportJson/exportCsv emitted tools and shell-commands but no MCP section, and the tools list is built via extractCoreTools, which deliberately strips mcp__ names. So even with mcpBreakdown populated, exports showed nothing MCP.

Change

Add an mcp section (server -> calls, with share %) to the JSON export and an mcp.csv to the CSV export, sourced from session.mcpBreakdown. Mirrors the existing tools section; README updated.

Schema string stays codeburn.export.v2 since this is purely additive (new keys, no changes to existing fields) and shouldn't break consumers pinned to v2.

Validation on real local data

$ codeburn export --format json --provider codex (June)
mcp: [{"Server":"node_repl","Calls":5,"Share (%)":100}]
tools: [{"Tool":"Bash",...},{"Tool":"Edit",...}]   # MCP correctly excluded here

New tests: exportJson emits per-server mcp rows with correct shares; exportCsv writes mcp.csv. Full export + codex + export-date-range suites pass (26/26), tsc --noEmit clean.

Out of scope

mcpInventory (which MCP/skill tools were available, vs used) is still empty for Codex because extractMcpInventory only understands Claude's deferred_tools_delta shape, not Codex tool_search_output events. Tracked as a follow-up.

Fixes #496