fix(exec/resume): require explicit selector and errors

[hermia-ai]

Summary

• require an explicit resume selector (SESSION_ID or --last)
• return clear errors when session ID is not found or when --last has no sessions
• add regression tests for selector-required and not-found cases

Scope

• code-rs/exec/src/lib.rs
• no vt100 snapshot deltas versus origin/main (already identical)

Validation

• pre-release passed in a clean worktree using a non-destructive mount-namespace env fix to avoid host /usr/bin/claude interfering with local-fallback test behavior