fix(acp): validate workspace access before session admission

[Kevin4234]

Background

When a conversation workspace was deleted, moved, or became inaccessible, the ACP adapter still attempted to use it as the working directory and failed with:

failed to start ACP adapter npx.cmd: The directory name is invalid

This made a workspace problem appear to be an npx.cmd or Agent environment failure.

Changes

• Validate the conversation workspace before creating, rerunning, continuing, recovering, or sending a session:
  • The path exists
  • The path is a directory
  • The directory is accessible
• Return structured errors:
  • workspace.path-not-found
  • workspace.path-not-directory
  • workspace.path-inaccessible
• Include projectId and workspacePath in error parameters
• Add localized Chinese and English error messages
• Update the product design and ACP MVP planning documentation
• Add Rust and frontend localization regression tests

Behavior

• Validation occurs before admitting a Run, Attempt, or ACP prompt turn
• Existing conversation history and composer input are preserved on failure
• Agent doctor status remains unchanged
• History reads, stop operations, and workspace listing remain available
• The ACP adapter process-launch boundary is unchanged
• No Windows error-code special cases or process-boundary fallback were added

Verification

• Rust workspace admission tests: 5 passed
• Frontend error localization tests: 9 passed
• cargo fmt --all --check
• TypeScript type checking
• Vite production build
• Browser verification of the conversation entry screen