fix: correct context loading from session new/overwrite and resume

[lamchau]

This fix corrects session handling with the overwrite session prompt.

Previously overwriting a session didn't

1. clear the context (which we load at init) https://github.com/block-open-source/goose/blob/7749b8aaf4390a83068045dd98f63345ed9566d9/src/goose/cli/session.py#L95
2. clear the session file when we write immediately (feat: auto save sessions before next user input #94)
3. missing test case when adding guards (feat: add guards to session management #101)
4. edge case when resuming a session (fix: session resume with arg handled incorrectly #145)

Steps to reproduce

$ uv run goose session start aaaa
PLEASE NOTE: the ollama provider is experimental, use with care
Session already exists at /Users/lam/.config/goose/sessions/aaaa.jsonl.
Would you like to overwrite it?

  [y]es      Overwrite the existing session
  [n]o       Pick a new session name
  [r]esume   Resume the existing session

Enter your choice: y
Overwriting existing session
session size: 4

The expected session size should be 0 (or empty).