fix(session): restore task context and bundled skill discovery

[xiami762]

Summary

• restore no-directory requests to the process working directory and model Tasks as ordinary sessions instead of a virtual default project
• discover bundled source skills independently of the request working directory and scope the skill cache by runtime context
• delete a project's registered sessions together with its registry entry while preserving the project directory and files
• retain the latest dev branch project-sharing and read-only behavior

Root cause

PR #553 routed directory-less requests through a virtual default worktree. Skill discovery then scanned that worktree instead of the Flocks source tree, so bundled skills disappeared when the server handled ordinary task requests.

Verification

• 67 project, middleware, and skill tests passed
• 61 session route tests passed
• 88 Session page and useSessions tests passed
• Ruff and ESLint passed
• TypeScript and Vite production build passed