fix(storage): keep session task metadata out of project workspaces

[kunaaal13]

Summary

Default session task metadata is stored inside the active project workspace:

<workspace>/.pi/tasks/tasks-<sessionId>.json

This is runtime state, not project content. It creates hidden files in every project where pi-tasks runs and couples task persistence to workspace-local storage.

Expected behavior

Store default session-scoped task metadata under the global pi data directory, namespaced by active workspace and session:

~/.pi/tasks/sessions/<project-key>/tasks-<sessionId>.json

<project-key> should be derived from the active session workspace path, not the host process cwd.

Requirements

• Use ExtensionContext.cwd as the workspace identity, following fix(storage): resolve project paths from session cwd #45.
• Keep session IDs isolated.
• Preserve project/shared task scope semantics.
• Preserve PI_TASKS named, absolute, and relative overrides.
• Resolve relative overrides from ExtensionContext.cwd.
• Keep global task defaults in the existing global config location.
• Define migration behavior for existing <workspace>/.pi/tasks files.
• Avoid storing task metadata in repositories by default.

Context

Related to #45 and #56. #56 fixed completed-list accumulation and session-file cleanup, but the storage location remains workspace-local.

Acceptance criteria

• New default session tasks write outside the active repository.
• Different workspaces cannot collide.
• Resume, fork, and new-session behavior remains isolated and functional.
• Existing storage override behavior remains backward compatible.
• Tests cover cwd differing from process.cwd, workspace namespacing, resume/fork, and overrides.

[kunaaal13]

@tintinweb created the issue

[kunaaal13]

@tintinweb can i pick this up? it is v annoying everyhwere i run pi it creates .pi folder.

[tintinweb]

I'll take a look. merging a couple of other things in the other extensions first.