fix: make range begin resolve and persist immutable commit baselines

[chtnnh]

Problem

know-code range begin --from <invalid-ref> can succeed because merge-base failures fall back to the supplied string. Later range commands then treat Git failures as empty history/tree state. Unborn repositories also fail through an implicit HEAD assumption.

Scope

Validate --from as a commit, resolve it to an OID, require a real merge base, and store only immutable OIDs in the session. Define and test the intended unborn-repository behavior.

Acceptance criteria

• Invalid, non-commit, and missing refs fail before writing a range session.
• A valid ref is persisted as its resolved commit OID.
• Git lookup failures cannot silently become an empty range or empty tree.
• Tests cover invalid refs, ref movement after begin, and the selected unborn-repo behavior.