FN-7794: apply fallback model's own thinking level when swapping in at runtime

Adds fallbackThinkingLevel plumbing so, when Fusion swaps from a primary model to a configured fallback model (executor, validator/reviewer, merger, planning, title-summarizer, heartbeat, and workflow-step lanes), the fallback's own configured thinking level is applied instead of silently reusing the primary lane's level.

- Add fallbackThinkingLevel option to AgentRuntimeOptions (agent-runtime.ts), AgentOptions (pi.ts), and ReviewOptions (reviewer.ts)
- Add per-lane resolvers: resolveExecutorFallbackThinkingLevel, resolvePlanningFallbackThinkingLevel, resolveValidatorFallbackThinkingLevel, resolveTitleSummarizerFallbackThinkingLevel, resolveMergerFallbackThinkingLevel (agent-session-helpers.ts), each following fallback-provider precedence and falling back to the primary lane/default thinking level when unset
- Export new resolvers from packages/engine/src/index.ts
- Apply the resolved fallback thinking level in createFnAgent's applyThinkingLevelIfSupported once a session has swapped to the fallback model (pi.ts)
- Wire fallbackThinkingLevel through executor session creation (workflow-step, task validator, child-agent, and main executor session paths), merger session creation, and heartbeat session creation
- Promote the fallback thinking level alongside the fallback model/provider when the no-visible-key Grok CLI fallback is promoted to primary, so the cleared fallback pair doesn't leave the session on the superseded primary's thinking level
- Route workflow-step fallback thinking level by which fallback candidate (validatorFallback vs globalFallback) actually matched
- Document fallbackThinkingLevel runtime-swap behavior in docs/settings-reference.md
- Add minor changeset for @runfusion/fusion
- Add regression tests covering fallback thinking-level resolution and application (agent-session-helpers.test.ts, pi.test.ts) and a shared test helper (executor-test-helpers.ts)

Files changed:
 .changeset/fn-7794-fallback-thinking-level.md      |   7 ++
 docs/settings-reference.md                         |   3 +
 .../src/__tests__/agent-session-helpers.test.ts    |  38 ++++++
 .../engine/src/__tests__/executor-test-helpers.ts  |  23 ++++
 packages/engine/src/__tests__/pi.test.ts           | 136 +++++++++++++++++++++
 packages/engine/src/agent-heartbeat.ts             |   3 +-
 packages/engine/src/agent-runtime.ts               |   5 +
 packages/engine/src/agent-session-helpers.ts       |  54 ++++++++
 packages/engine/src/executor.ts                    |  31 ++++-
 packages/engine/src/index.ts                       |   5 +
 packages/engine/src/merger.ts                      |   7 +-
 packages/engine/src/pi.ts                          |  16 ++-
 packages/engine/src/reviewer.ts                    |   6 +
 13 files changed, 327 insertions(+), 7 deletions(-)

Fusion-Task-Id: FN-7794

Fusion-Task-Lineage: c94d621a-ccbd-42b2-9fe6-cb619418ad90

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

docs/settings-reference.md

@@ -65,6 +65,9 @@ Defaults from `DEFAULT_GLOBAL_SETTINGS`; key scope from `GLOBAL_SETTINGS_KEYS`.
 | `fallbackProvider` | `string` | `undefined` | Fallback provider when the selected/default model hits transient provider failures or model-compatibility/auth-tier rejections. Dashboard chat also offers this fallback for explicit user-selected models, but the engine only swaps for retryable provider/model-selection failures. |
 | `fallbackModelId` | `string` | `undefined` | Fallback model ID (must pair with `fallbackProvider`). |
 | `fallbackThinkingLevel` | `ThinkingLevel` | `undefined` | Optional global fallback-lane thinking override for the `fallbackProvider`/`fallbackModelId` pair. Inherits `defaultThinkingLevel` when unset. |
+
+Fallback thinking-level values are applied at runtime when Fusion swaps from the primary model to the configured fallback model; if unset, the active lane/default thinking level continues to apply.
+
 | `defaultThinkingLevel` | `"off" \| "minimal" \| "low" \| "medium" \| "high" \| "xhigh"` | `undefined` | Default reasoning effort for AI sessions. `xhigh` requests maximum reasoning effort; Claude CLI adapters map it to `high` for non-Opus models and `max` for Opus models. If a provider/runtime rejects simultaneous `thinking` and `reasoning_effort` parameters, Fusion retries without the explicit thinking override instead of failing the run. |
 | `ntfyEnabled` | `boolean` | `false` | Enable ntfy push notifications. |
 | `failureNotificationMode` | `"sticky-only" \| "terminal-only" \| "all"` | `"sticky-only"` | Failure notification behavior. `sticky-only` defers failed-task notifications by `failureNotificationDelayMs` and suppresses transient self-recoveries. `terminal-only` suppresses while auto-retry is still active and only dispatches when `paused === true` or `column === "in-review"` with `status === "failed"`. `all` restores legacy immediate failure notifications. |