refactor(config): remove deprecated safer shell flag from v15

[dgageot]

The safer boolean on the shell toolset has been deprecated and ignored for some time: once the runtime started classifying every shell command natively via pkg/safety and gating execution through the session's safety mode, the flag became redundant. This PR removes it from pkg/config/latest (config version 15), which is the only version still open to modification.

Frozen versions v11–v14 are untouched and continue to accept (and silently ignore) the field. For the upgrade path, a version: "14" YAML with safer: true still loads correctly — the v14→v15 migration via types.CloneThroughJSON drops the unknown field. A version: "15" YAML with safer: true is correctly rejected under strict decoding with an unknown field "safer" error.

The change drops Toolset.Safer and its validation check from pkg/config/latest, removes the matching test cases, removes the safer property from agent-schema.json, and updates docs/tools/shell/index.md to drop the option row and note the removal.