Add configurable default attempts count (like effort levels)

[that-github-user]

Summary

Currently --attempts defaults to 3. Users should be able to set a persistent default so they don't have to pass -n 5 every time. Similar to how Claude Code allows "effort" selection that persists.

Proposed design

Config file: .thinktank/config.json

{
  "defaults": {
    "attempts": 5,
    "model": "sonnet",
    "timeout": 300,
    "runner": "claude-code"
  }
}

CLI override hierarchy

1. CLI flag (--attempts 3) — highest priority
2. Config file (.thinktank/config.json) — project-level
3. Built-in defaults (3) — fallback

Commands

• thinktank config set attempts 5 — set a default
• thinktank config get attempts — view current default
• thinktank config reset — restore built-in defaults

User request context

"maybe we can have it be in some way like claude-code allows effort selection that maintains throughout once set unless manually overridden"

Acceptance criteria

• .thinktank/config.json loaded on startup
• CLI flags override config file values
• thinktank config subcommand for viewing/setting defaults
• Config file created on first config set (not on install)

[that-github-user]

Fixed in #102. Added .thinktank/config.json for persistent defaults + thinktank config set/get/list commands. 26 new tests. Generated by thinktank Opus.