Keep stored session model on resume unless flags override

[TheGreatAxios]

Summary

• Resume restores the stored session's provider:model instead of clobbering it with the launch default
• Explicit --provider/--model flags win, tracked as a parse-time signal rather than inferred by value comparison
• Legacy model-less run.json records keep the launch default; malformed stored identities never throw

Verification

• session-start + resume-seed suites pass (12 pass, 0 fail)
• config/session/tui suites pass (2736 pass, 0 fail)
• bun run build passes; lint clean; typecheck shows only the pre-existing vendor semver error (identical on clean tree)
• manual spot-check: loadConfig sets modelOverride only with --model/--provider; resume restores stored-p:stored-m on picker and id branches, keeps launch default with flags or legacy records
• Full suite: 8593 pass, 2 fail — both pre-existing/environmental (missing ts-prune binary; shell-guard timing test passes in isolation)

Fixes CL-8995

[linear-code]

CL-8995