feat: autocomplete framework from user's Organization on assistant creation

[martinydeAI]

Resume

Description

On the assistant-creation form, pre-fill the framework field from
the current user's organization.defaultFramework. Eliminates the
"every user re-types the same thing" problem that motivated ADR 005
(#65).

Tasks

• Resolve the current user's organisation in the assistant-creation form / controller.
• Default framework to organization.defaultFramework on a fresh form (still editable by the user).
• If the user has no organisation, fall back to the previous behaviour (empty / manual).
• Integration tests covering: org with default → field pre-filled, no org → no pre-fill, user override on submit honoured.

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Details - AI specificities

• Blocked by the Organization entity issue AND the
  User → Organization relation issue. Pick up once both have landed.
• ADR 005 specifies the value is stored on the assistant, not read
  live — the autocomplete writes into the form's submitted value, it
  does not turn Assistant.framework into a derived/computed property.
• Language model is not autocompleted from the organisation. ADR 005
  intentionally does not model a default LM.