feat(agent): select a delegated identity for hook recording

[geekgonecrazy]

What

atomic agent identity set <name> makes every hooked agent on this machine record turns under a delegated identity's own key instead of the plus-tag of the default identity. One identity per machine — deliberately not per repository — persisted to a new top-level agent_identity in ~/.atomic/config.toml.

New command tree under atomic agent identity:

• set <name> — validates eagerly: the identity must resolve in the store and be an agent/delegated type (human identities are refused — signing agent work with the human's key is worse than the honest plus-tag), and a missing delegation certificate prints a warning.
• unset — clears the selection; hooks go back to the plus-tag path.
• show — the effective identity, its source, email, key, and active delegation URN — the same resolution the hooks run.

Resolution order (per hook invocation)

ATOMIC_AGENT_IDENTITY env var
  > global agent_identity setting
  > active server profile's agent_identity binding
  > plus-tag fallback (behavior from before agent identities existed)

The server-profile link closes a promise already written down: bind_agent_identity documents its binding as "so hooks use it by default", but only push auth ever read it — the recording path never did. Now creating an agent identity makes hooks use it with zero extra steps, and the global setting overrides it.

Design invariants

• Backward compatible: nothing configured anywhere → recorded turns are byte-identical to today (plus-tag author, human key, no envelope URN).
• atomic record untouched: the human path keeps using the default identity.
• Recording never fails over identity selection: a mistyped name degrades to the plus-tag path with a debug log; eager validation lives in set, soft fallback lives in the record path.
• atomic-agent never reads config: the resolved name crosses as plain data through TurnRecordOptions.agent_identity into build_agent_author and active_delegation_urn, so change headers carry the agent's public key and envelopes name the active delegation certificate.

Also

• atomic agent status shows the effective identity in human output and as agent_identity: {name, source} in --json (stable source keys: env, global, server-profile).
• Intent: ATOM::aaron::15 (vault).

Testing

• cargo test -p atomic-config -p atomic-agent -p atomic-cli — all pass (1,356 / 1,927 / 28), including new unit tests for the resolution order, blank-value skipping, keyed-attribution threading, and never-fail fallback.
• Clippy clean for all touched files.
• Manually exercised the built binary against an isolated config dir: set/unset/show round-trip, human identity refused, unknown identity → IdentityNotFound, env var precedence over the global setting, status human + JSON output.