Function is_valid_id 

pub fn is_valid_id(s: &str) -> bool

T-310: charset rule for project.id and agent ids — both flow, unquoted, into shell-bound strings ({project}:{agent} in supervisor::build_up_command) and into tmux session names. A conservative ASCII allowlist keeps every downstream consumer safe at the boundary instead of forcing each call site to quote defensively. : is intentionally NOT allowed — it’s reserved as the canonical <project>:<agent> join separator everywhere in teamctl, and admitting it into an id would make id() parse ambiguous.

Rejected by construction: any whitespace, any shell metacharacter (;, |, &, $, backtick, (, ), <, >, *, ?, ~, !, #, ', ", \), any control char, any non-ASCII (incl. emoji), the empty string.