fix: preserve Step thinking effort preferences

[MelodyVAR]

Step's startup and model-selection hooks always reselected the highest supported thinking effort after the session had already applied CLI and saved preferences. For models supporting low/medium/high, both step --thinking medium and a saved medium default were overwritten with high.

Keep the session-resolved effort when enriching model capabilities, clamp only unsupported choices, and ignore late discovery results for a model that is no longer active. The thinking selector now marks the effective saved default using per-model/global precedence and capability clamping.

With no configured preference, Step follows the SDK default (medium, clamped to model capabilities). Enter and /effort medium remain session-scoped; Ctrl+S still saves a default.

Validation:

• 99 coding-agent tests passed, including startup, disk-persisted defaults, model selection, pending-discovery user changes, and stale-model discovery.
• 18 CLI tests passed, including the default marker.
• pnpm run build:offline passed.
• All checks from pnpm run check passed, with Biome run without --write.