[bug] fix plugin reload and session meta patching

[shayne-snap]

Summary

Fix two state handling bugs in Whale:

• return ApplyLoadedConfig errors from plugin disabled-config reloads instead of silently updating runtime plugin state from invalid config
• introduce SessionMetaPatch so TotalCostUSD and TurnCount can be explicitly reset to zero without losing the existing partial-patch semantics

Root Cause

reloadPluginDisabledConfig ignored the error from ApplyLoadedConfig, unlike the skill reload path. PatchSessionMeta used numeric zero as the sentinel for "not provided", which made it impossible to patch cost or turn count back to zero.

Validation

• go test ./internal/session ./internal/app
• make test
• git diff --check
• manual TUI smoke with ./bin/whale: /plugins surfaced invalid ui.view_mode and /memory remained available after the failed reload; a normal turn wrote session meta with turn_count: 1 and cost