feat: improve Session API ergonomics with convenience constructors

[xpcmdshell]

Summary

Improves the developer experience for Session initialization based on API ergonomics review.

• Make storage parameter honestly required (type hint was lying - said Optional but raised at runtime)
• Add Session.from_base(path) convenience constructor for local dev
• Add Session.subprocess(), Session.in_process(), Session.container() preset constructors
• Re-export executor types at py_code_mode top level (reduces import scatter)

New API

from py_code_mode import Session

# Simplest - just a path (in-process, auto-discovers tools/skills/artifacts/requirements.txt)
async with Session.from_base("./.code-mode") as session:
    await session.run("tools.list()")

# With options
async with Session.from_base(
    "./.code-mode",
    timeout=60.0,
    extra_deps=("pandas",),
    allow_runtime_deps=False,
) as session:
    ...

# Explicit constructors for full control
Session.subprocess(storage_path="./data", tools_path="./tools")
Session.in_process(storage_path="./data")
Session.container(storage_path="./data", image="my-image:latest")

Breaking Changes

• storage parameter type changed from StorageBackend | None to StorageBackend
  • Was already required at runtime (raised ValueError if None)
  • Now the type signature is honest

Testing

All 56 session tests pass.