cli: add --init-skip-if-exists for anonymous dev init

[nicu-chiciuc]

Summary

• Adds a new convex dev flag: --init-skip-if-exists
• Threads this option through:
  • dev CLI options
  • configure flow
  • anonymous local deployment initialization
  • doInitConvexFolder(...)
• Adds skipIfExists?: boolean support to doInitConvexFolder(...)
• Adds tests for init behavior in src/cli/lib/codegen.test.ts:
  • preserves existing convex/README.md and convex/tsconfig.json when skipIfExists is enabled
  • still creates missing files
  • preserves existing default overwrite behavior when option is not set

Motivation

In anonymous local dev flows (e.g. CONVEX_AGENT_MODE=anonymous npx convex dev --once), users may want to avoid overwriting existing convex/README.md and convex/tsconfig.json in worktrees.
This change provides an opt-in guard without changing existing defaults.

Usage

CONVEX_AGENT_MODE=anonymous npx convex dev --once --init-skip-if-exists

[nsxdavid]

Very useful!

[dowski]

Hey @nicu-chiciuc - thanks for the contribution! This is honestly a good change to make default I think. Basically, unless the CLI is creating the convex/ directory from scratch, it shouldn't touch the tsconfig.json or README.md files in there.

Are you up for stripping down the PR functionality (no more flag) to align with that?

[dowski]

This is honestly a good change to make default I think.

And for clarity, I mean however the command is executed (not limited to CONVEX_AGENT_MODE=anonymous).