Refactor: rename "toolkit" to "ptoolkit"

[rolandwalker]

Description

Refactor: rename "toolkit" to "ptoolkit" when referring to prompt_toolkit extensions and utilities.

When first reading the layout I assumed that toolkit referred to some generic, rather than specific, toolkit for mycli. This just helps keep things straight.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Low: potential backward-compat break from module rename without shim

• Changed imports now point to mycli.packages.ptoolkit (for example key_bindings.py:19, main.py:96), and mycli/packages/toolkit/* was renamed to mycli/packages/ptoolkit/*.
• If any external plugin/user code imports mycli.packages.toolkit.*, this PR will break them at import time.
• Action: add a temporary compatibility package mycli/packages/toolkit/ that re-exports from ptoolkit, or explicitly call this out as a breaking change in release notes.

2. Missing regression test coverage for import-path refactor

• This PR is a pure import/module-path rename, but there is no test asserting those critical imports still resolve (e.g. startup path via mycli.main, key binding history search path).
• Action: add a small smoke test that imports mycli.main and executes key-binding module import path resolution, so future refactors catch ModuleNotFoundError regressions early.

Security concerns found in this PR: none.

Validation note: I could not run tests in this environment because pytest is not installed.