revert(inquirerer): simplify keypress to per-instance model

[pyramation]

Summary

Reverts the SharedInputState/stack-based ownership approach back to a simple per-instance listener model. The complex multi-instance handling from PR #29 and #31 didn't work as expected during manual testing, so this PR simplifies back to the original approach where each TerminalKeypress instance manages its own data listener independently.

Changes:

• Remove SharedInputState, WeakMap, and stack-based ownership logic from keypress.ts
• Restore simple per-instance data handler with active boolean flag
• Keep handler clearing in pause() to prevent handler accumulation between questions
• Add JSDoc documentation explaining the single-instance limitation
• Add README section with best practices for managing multiple Inquirerer instances

The simplified approach is more predictable. Users should either reuse a single Inquirerer instance or call close() on one before creating another.

Review & Testing Checklist for Human

• Manual test interactive prompts - Run a CLI that uses list/checkbox/autocomplete prompts and verify arrow keys work correctly without double-firing
• Test multiple questions in sequence - Verify that asking multiple questions in a row (same instance) doesn't cause handler accumulation
• Test the documented close() workflow - If you need two separate Inquirerer instances, verify that closing the first before using the second works as documented

Recommended Test Plan

Run create-gen-app or another CLI that uses inquirerer interactively and verify:

1. Arrow key navigation works correctly in list prompts
2. Multiple prompts in sequence work without issues
3. Ctrl+C exits cleanly

Notes

• This is a revert/simplification, not a fix for multi-instance support
• The README now documents that multiple concurrent instances will cause duplicate keypresses
• All 90 inquirerer tests and 28 create-gen-app tests pass

Link to Devin run: https://app.devin.ai/sessions/48555739db3d4f1b9e9edfa9ea1e559a
Requested by: Dan Lynch (@pyramation)

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