Add per-message timings

[brandonpike]

Introduce the /timing local command for CLI.

I found myself curious how long a specific message was taking to process, so I added a way for users to get this information out at the end of each user message.

If enabled, the /timing command enables you to get timings out at the end of each message. The aggregates still apply on /cost.

Example:

> can you just give me a one sentence summary of everything?
...
[System] Task completed [success=true]: Provided a single sentence summary of tycode-core: It is the core engine for an AI coding assistant that orchestrates Claude conversations through specialized agents, executes tools (file ops, builds, type analysis, sub-agents), manages workspaces with security, persists sessions, and integrates with MCP servers.

> /timing
[System] Timings: enabled

> can you just give me a one sentence summary of everything?
...
[System] Task completed [success=true]: Provided one sentence summary of tycode-core.
[System] Timing => Human: 6.6s, AI: 8.5s, Tools: 0.0s, Total: 15.1s

> /cost
[System] === Session Cost Summary ===
...
Time Spent:
  Waiting for human:   67.4s
  AI processing:      198.8s
  Tool execution:       0.1s
  Total session:      266.2s

[brandonpike]

unrelated: I noticed local commands don't show up in /help. I started fixing it, but not relevant for this PR.

[brandonpike]

A thought about naming:

Could rename this to Timing and rename TimingStat to TimingStats. Fits a bit better, as current_state and state_start aren't really fitting to be under a name TimingStats.