Add Pi package and local extension management

[ScottRBK]

Adds persistent package provisioning so callers can install Pi tools before starting agent runs.
The shared API exposes add_package, list_packages, and remove_package, with Pi as the first
implementation and clear unsupported-operation errors from other adapters.

• Supports exact npm versions, Git sources with explicit refs, and local extension files or
  package directories. Local sources stay in place and survive removal of their registration.
• Uses Pi's existing user configuration and native install/remove commands. Fresh AgentShell
  instances load the persisted packages; callers retain responsibility for container isolation.
• Validates sources and settings, handles timeouts/cancellation, and verifies persistence because
  Pi can exit successfully even when saving settings fails.
• Documents usage and scope. Per-run resource selection and individual toggles are deferred.

Validation: the full unit/integration suite passed (1,119 tests), followed by all 47 focused package
tests after adding one further CI regression case. Five real Pi 0.85.1 smoke tests passed, covering
local registration, removal, loading by a fresh shell, symlink identity, and settings-write failure.
The smoke tests need no model requests or downloads. npm/Git command handling is covered with mocked
subprocesses.

Independent Astra review at x-high reasoning found one P2 issue: following symlinks changed Pi's
local package identities. The fix preserves link paths, with regression coverage for both linked
configuration directories and extension files. The reviewer verified the fix against real Pi and
reported no remaining actionable findings.

Closes #22.

[ScottRBK]

can you please move the package management specific documentation to ./docs/development/package_management.md and add a pointer to it in the AGENTS.md - this is way too much bloat in an agents.md file