Resolve the effective main-agent recursion_limit.
Resolves runtime.recursion_limit through the standard env → config.toml
→ default precedence. An out-of-range value (below RECURSION_LIMIT_FLOOR
or above RECURSION_LIMIT_CEILING) is discarded with a logged warning and
the next lower-precedence layer is tried, so a bad higher-precedence
override cannot mask a valid TOML setting (or the default).
