feat: log workspace file tool outcomes

[bonigo1998]

Adds tool_start, tool_success, and tool_error events for read_file calls executed by AgentLoop.

Records the model step, tool name, byte count, and error reason. Distinguishes file-read failures from oversized contents withheld from the model.

Validation:

• Application builds and both existing CTest suites pass.
• Successful read logged 103 bytes.
• Missing-file case logged read_failed.
• A 4,001-byte file logged file_too_large with the correct byte count.

These events cover model-requested file reads. The separate /read and /askfile commands are not instrumented.