[19.0][MIG] mail_activity_done: Migration to 19.0

[shamnaskoyani]

Summary

Migration of mail_activity_done from 18.0 to 19.0.

Changes:

• Bump module version to 19.0.1.0.0.
• Remove the legacy migrations/18.0.1.0.0/ directory. That script was specific to the 17.0 → 18.0 reshuffle (when the standalone done boolean column on mail.activity was dropped in favour of using active + keep_done on the activity type). It is unreachable from a 19.0 install and would never run again.
• Drop test_migration, which exercised the now-removed script.
• Regenerate README.rst and static/description/index.html via pre-commit (oca-gen-addon-readme).
• Add readme/CREDITS.md and credit the migration contributor in readme/CONTRIBUTORS.md.

No model, view, controller, JS, or hook changes were required — the only Python code in the module is a single keep_done = fields.Boolean(default=True) field on mail.activity.type and a post_init_hook running a single UPDATE statement. None of the 19.0-breaking patterns (groups_id → group_ids, read_group → formatted_read_group, _() → self.env._(), self._context → self.env.context) are present.

Test plan

• Install mail_activity_done on a fresh 19.0 database — confirm keep_done = True is set on existing mail.activity.type records by the post-init hook.
• Create a new mail.activity.type — confirm keep_done defaults to True.
• Run pytest/odoo --test-tags mail_activity_done — confirm TestMailActivityDone passes.
• Pre-commit clean (verified locally).

Module maturity

Beta (matches the pre-19.0 maturity).

Closes the mail_activity_done entry on #85.