feat(tui): add /effort command to select the model's reasoning level

[Sayt-0]

What

New /effort slash command to select the reasoning-effort level of the current model directly, complementing the existing Shift+Tab cycling. Available in both the full TUI and the lean TUI.

Input	Behavior
/effort high	Sets the level (session override, never persisted to YAML)
/effort	Shows usage: none, minimal, low, medium, high, xhigh, max
/effort turbo	Error listing valid level names
/effort max on gpt-5	Error listing the levels this model supports
Non-reasoning model / remote runtime	Informational message (same as Shift+Tab)

How

• New Runtime.SetAgentThinkingLevel(ctx, agent, level) interface method
• Cycle + set share one path: applyAgentThinkingLevel (reasoning check → supported levels via modelinfo.SupportedThinkingLevels → provider re-creation incl. alloy pools → override install)
• RemoteRuntime → ErrUnsupported (server owns model config)
• App.SetAgentThinkingLevel refreshes agent info so the sidebar updates
• Success surfaces a toast (full TUI) / notice + status bar update (lean TUI)

Flow

/effort high
   └─ SetThinkingLevelMsg ─ handleSetThinkingLevel (TUI)
        └─ App.SetAgentThinkingLevel
             └─ Runtime.SetAgentThinkingLevel
                  ├─ model reasons? ──no──▶ ErrUnsupported
                  ├─ level supported by model? ──no──▶ error + supported list
                  └─ recreate provider(s) with thinking_budget ─▶ model override

Tests

Area	Coverage
pkg/runtime	Valid level installs override · unsupported level rejected with supported list · non-reasoning model → ErrUnsupported · per-model top tier (max on Opus 4.7) accepted
pkg/tui/commands	/effort high and bare /effort parsing
pkg/leantui	Set path updates status · unknown level rejected
Mocks	All 5 Runtime mocks updated

Docs

• /effort row in the TUI slash-command table (docs/features/tui)
• /effort mentioned alongside Shift+Tab in the thinking guide (docs/guides/thinking)

Validation

• task build
• golangci-lint run — 0 issues
• Tests pass on all touched packages (runtime, app, cli, tui/..., leantui)