[Python] Identity system (~/.fd5/identity.toml)

[gerchowl]

Parent: #161

Goal

Create fd5.identity module for loading and managing user identity from ~/.fd5/identity.toml.

Spec

• Identity dataclass: type (orcid/github/email/anonymous), id (string), name (string)
• load_identity() -> Identity — read from ~/.fd5/identity.toml, return anonymous if missing
• save_identity(identity: Identity) — write to ~/.fd5/identity.toml
• identity_to_dict(identity: Identity) -> dict — for embedding in audit entries
• Support for type values: orcid, github, email, anonymous

TOML format

[identity]
type = "orcid"
id = "0000-0002-1825-0097"
name = "Lars Gerchow"

TDD

• Test load from valid TOML file
• Test load returns anonymous when file missing
• Test save + load round-trip
• Test validation of identity types
• Test ORCID format validation (XXXX-XXXX-XXXX-XXXX)