Add bench/arm.ts, the per-arm scaffolding that produces the single arm
definition the runner consumes. Every arm shares an identical task-agnostic
base prompt and the same repository coordinates and token; each arm then
receives a minimal, symmetric bootstrap.
The deliberate asymmetries follow the shipped products: the gitea-axi arm
embeds the bundled Agent Skill (its body, charging its ambient cost to
gitea-axi); the tea and raw-api arms get a one-line native-discovery pointer;
the gitea-mcp arm runs with the shell disabled and only the MCP server
attached, its dispatcher schemas loading eagerly. Shell arms' PATH and guard
come from bench/guard.ts.
