add: document call.timeElapsed hook for assistant hooks

[dhruva-reddy]

Description

• Added documentation for the call.timeElapsed assistant hook event, which was implemented in the codebase but missing from docs
• Updated the supported events list to include call.timeElapsed
• Updated the "How hooks work" section to mention call.timeElapsed in the options description
• Added three example subsections:
  • Basic usage: simple time-based notification
  • Call discipline (wrap-up and graceful close): multi-stage pattern combining call.timeElapsed hooks with maxDurationSeconds for structured call ending
  • Inject a system message to guide the LLM: using message.add action to change LLM behavior at a time threshold
• Added "Enforce call duration limits with graceful wrap-up behavior" to the common use cases list
• File modified: fern/assistants/assistant-hooks.mdx

The call.timeElapsed hook is fully implemented in libs/core/src/types/call.types/call.hooks.types.ts and scheduled at runtime via hookStream.ts. It fires once at a specified number of seconds (1-3600) from call start. This documentation fills the gap so customers can discover and use this feature.

Testing Steps

• Run fern check -- passes with 0 errors (4 pre-existing warnings)
• Verify JSON examples are valid -- all new JSON blocks parse correctly (1 pre-existing invalid block in existing content, not introduced by this PR)
• Verify internal links resolve -- no new internal links added (2 pre-existing broken API reference hash links, not introduced by this PR)
• Confirm new content appears in the correct position (after "Handle customer speech timeout", before "Handle low confidence transcripts")
• Review page content for style guide compliance (active voice, present tense, no marketing language)
• Cross-checked options.seconds field name, min/max values (1-3600), and one-shot behavior against source types in call.hooks.types.ts

[github-actions]

🌿 Preview your docs: https://vapi-preview-b711377c-920c-4f87-b92b-adbf2b950f48.docs.buildwithfern.com