feat: cross-session execution history (persistent tracking)

[JeremyDev87]

Purpose

Track execution history across sessions — what was done, by which agent, cost, duration, outcome. Currently context.md is session-scoped and lost on context clear.

Changes

• Persistent storage (SQLite or JSON file under .codingbuddy/)
• Record: session ID, timestamp, mode, agent, task, cost estimate, outcome, PR numbers
• Query API via MCP tool: "show last 10 sessions", "total cost this week"

Acceptance Criteria

• Execution records persist across sessions
• Records include: timestamp, agent, task summary, files changed, PRs created, estimated cost
• MCP tool to query history
• Auto-cleanup of records older than configurable retention period

[JeremyDev87]

Superseded by #827 (plugin persistent SQLite DB). Plugin provides natural persistence independent of MCP server lifecycle.