feat(zerocode): add mid-session agent switching (Ctrl+G)

[JordanTheJet]

Summary

• Base branch: master
• What changed and why: In the zerocode TUI the agent was chosen once at session start and could not be changed without quitting and relaunching — Ctrl+N restarts the same agent and Ctrl+S only switches between existing sessions, so there was no path back to the agent picker from a live session. This adds a SwitchAgent action (default Ctrl+G) that opens an agent-picker overlay from an active session, mirroring the existing session-switch overlay; Enter starts a fresh session against the chosen agent, Esc returns to the current session untouched.
• Restart and switch now share start_fresh_session, which commits agent_alias only after the daemon confirms the new session — a failed create no longer strands the alias on an agent the live session isn't running.
• agent_overlay is mirrored into wants_text_input() so global keys (?, Ctrl+R) treat the new picker the same as the session picker regardless of any leftover draft; switching also warns via the info bar when a draft or queued messages are cleared instead of dropping them silently.
• Scope boundary: Does NOT change the startup agent picker, session switching (Ctrl+S), the Ctrl+N restart semantics, or any daemon/RPC surface. No copy/paste changes (tracked separately).
• Blast radius: Confined to apps/zerocode (TUI client). The shared restart_session_for_state was refactored to delegate to start_fresh_session; behavior for the existing Ctrl+N restart path is unchanged (it passes the current alias and the same "New session started." notice).
• Linked issue(s): None — addresses a direct user QOL request ("can't change agents easily").
• Labels: expect auto path/scope labels for apps/zerocode; suggest enhancement, size: S, risk: medium (touches apps/zerocode/src/**).

Validation Evidence (required)

cargo fmt --all -- --check
cargo clippy --all-targets -- -D warnings
cargo test

• Commands run and tail output:
  • cargo fmt --all -- --check → clean, no diff (exit 0).
  • cargo clippy --all-targets -- -D warnings → Finished \dev` profile [unoptimized + debuginfo] target(s) in 4m 03s`; no warnings.
  • cargo test → all suites pass (exit 0). The zerocode package: test result: ok. 348 passed; 0 failed; 0 ignored. Full-workspace run green (e.g. system::full_stack ok. 9 passed; 0 failed).
• Beyond CI — what did you manually verify? Traced the new key path end-to-end: input bar does not consume Ctrl+G (control chords return NotHandled, input_bar.rs:1052), so the chat handler opens the overlay; overlay captures keys (nav/Enter/Esc), Enter on the current agent is a no-op, click only moves selection (Enter confirms) so a stray click cannot discard the session. Confirmed start_fresh_session commits agent_alias only on the success branch and the WSS-ACP CWD-picker path carries the new alias without mutating live state. NOT verified: live WSS-ACP remote daemon switch and a real daemon session_new failure (the error-path rollback is covered by reasoning + the alias-on-success structure, not an integration test).
• If any command was intentionally skipped, why: None skipped.

Security & Privacy Impact (required)

• New permissions, capabilities, or file system access scope? No
• New external network calls? No
• Secrets / tokens / credentials handling changed? No
• PII, real identities, or personal data in diff, tests, fixtures, or docs? No
• If any Yes, describe the risk and mitigation: N/A.

Compatibility (required)

• Backward compatible? Yes
• Config / env / CLI surface changed? Yes — adds a new rebindable keymap action chat.switch_agent (default Ctrl+G) and two new info-bar strings. No migration required; existing configs and keybinding overrides are unaffected, and Ctrl+G was previously unbound.
• If No or Yes to either: exact upgrade steps for existing users: None — the new binding is additive and active by default.

Rollback (required for risk: medium and risk: high)

Low blast radius (single TUI binary). git revert <sha> cleanly removes the action, overlay, and the start_fresh_session refactor (the refactor is behavior-preserving for the existing restart path, so reverting restores the prior inline restart_session_for_state).