perf: speed up shell completion

[D3f0]

Summary

• lazy-load Copier only when create.package executes
• add INVOKE_COMPLETION_DISABLE_PLUGINS=1 to skip installed collection plugins during shell completion
• preserve project/local tasks, built-in -x completions, default plugin completion, and non-completion plugin loading
• document the behavior and add subprocess-level regression coverage

Performance

Measured over 20 real completion subprocesses in the development environment:

Command	Median
main: intk --complete -- intk -x xxx	486.1 ms
branch: same completion	460.7 ms
branch with INVOKE_COMPLETION_DISABLE_PLUGINS=1	314.3 ms

The lazy Copier regression also verifies that copier is absent from sys.modules after built-in completion.

Verification

• uv run nox: Python 3.11, 3.12, 3.13, and 3.14 sessions pass; each reports 768 passed and 18 skipped
• focused completion/Create tests: 10 passed
• Ruff checks and formatting: pass on changed files
• Pylint: 10.00/10 on changed Python files
• reviewer: no Critical, Important, or Minor findings

pyright src/invoke_toolkit and ty check src/invoke_toolkit still report pre-existing repository-wide typing errors unrelated to this diff.

Closes #143