Fix the plan-mode exit action type name in the fleet mode guide

[examon]

The "From plan mode" section of docs/features/fleet-mode.md introduces its snippet with "The
generated session event types describe it as:", and then labels the union PlanModeExitAction.
The generated session-event union is ExitPlanModeAction; the SDK does not export the
documented name, which appears nowhere in the repository outside that snippet and nowhere in the
published package.

This renames the identifier in that one snippet. The four string values were already correct.

Fixes #2089

The change

-type PlanModeExitAction =
+type ExitPlanModeAction =

One token, in docs/features/fleet-mode.md.

Why this identifier

• The sentence above the snippet says "generated session event types", and
  ExitPlanModeAction is declared in nodejs/src/generated/session-events.ts and re-exported
  as a type from the package root.
• The generated session-event payloads use it directly for actions, recommendedAction and
  selectedAction.
• The comment the snippet keeps for autopilot_fleet matches the one on that declaration.

Before and after

Against the published package (@github/copilot-sdk@1.0.8, TypeScript 5.9.3, strict,
moduleResolution: NodeNext, skipLibCheck):

# the identifier the guide showed
$ npx tsc --project tsconfig.json
old-name.ts(1,15): error TS2305: Module '"@github/copilot-sdk"' has no exported member 'PlanModeExitAction'.

# the identifier the guide now shows
$ npx tsc --project tsconfig.json
$ echo $?
0

A value outside the union is still rejected (error TS2322: Type '"autopilot_swarm"' is not assignable to type 'ExitPlanModeAction'), so the corrected name really does resolve to the
four-value union.

Checks

npm run extract && npm run validate:ts in scripts/docs-validation passes before and after
this change - the snippet declares its own alias, so it compiled either way. The four values and
their order match nodejs/src/generated/session-events.ts exactly.

Only docs/features/fleet-mode.md changes; no generated file, binding source, or public API is
touched.

[Copilot]

Pull request overview

Corrects the fleet mode guide to use the exported session-event type name.

Changes:

• Renames PlanModeExitAction to ExitPlanModeAction in the TypeScript snippet.

Show a summary per file

File	Description
docs/features/fleet-mode.md	Aligns the documented type name with the generated SDK export.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Medium