Read AGENT_PERSONA envvar as first-class identity input and select histories by persona name

[hai-pilgrim]

Context

AGENT_PERSONA is already read in one place (tui/src/pets/talking_signal.rs:66) but only to write the talking-file PID signal. It is not used as a first-class identity selector.

Meanwhile, conversation histories (thread-store, message-history) are keyed by thread ID and are not strongly bound to a persona name. A user running AGENT_PERSONA=pilgrim clanker and later AGENT_PERSONA=hai clanker in different directories should get separate, persona-scoped history — not directory-scoped or thread-ID-only history.

What is needed

1. AGENT_PERSONA as identity input. When set, this envvar should determine which persona is active for the session — selecting the character manifest, history partition, and TUI identity. It should be the canonical way for a launcher script or wrapper to tell clanker "you are X."

2. Directory-agnostic history selection by name. Histories must not be scoped to the working directory. A persona's conversation history should follow the persona name, not the $PWD. If I run AGENT_PERSONA=pilgrim clanker from /home/me/project-a and then from /home/me/project-b, I should see the same persona history (or at least have the option to).

3. CLI integration. The existing --name flag and AGENT_PERSONA envvar should converge on a single identity-resolution path: --name > AGENT_PERSONA > last-active (from CC-01 remainder: persisted clanker_id and bare last-active-character continuation #32) > default.

Relationship to existing issues

• CC-01 remainder: persisted clanker_id and bare last-active-character continuation #32 (persisted clanker_id) covers the persistence of which character was last active. This issue covers the input mechanism (AGENT_PERSONA envvar) and the history partitioning by name rather than directory.
• Character picker, status surface, and anonymous session adoption #33 (character picker) covers interactive selection. This issue covers non-interactive, environment-driven selection.
• CC-05 (in flight): clanker-character plugin — wizard capability, package activation, durable last-active journal #38 (CC-05, character plugin) may consume this as an input source for wizard/activation.

Acceptance criteria

• AGENT_PERSONA=<name> clanker selects the named persona (manifest, avatar, history)
• --name <name> takes precedence over AGENT_PERSONA
• Conversation history is retrievable by persona name regardless of working directory
• Existing thread-store and message-history paths do not break for users without AGENT_PERSONA set