Unified Agent Tuning (internal/agentcfg)

• agentcfg is the single owner of the harness-neutral model/effort surface and of the mapping down to each harness's native mechanism (claude/copilot --effort, codex -m + -c model_reasoning_effort, grok --reasoning-effort, pi --thinking, opencode's session-message model/variant, acpx --model for cursor/acp:<target>). Add a harness there, not in an adapter or in eval. rovodev and antigravity are deliberately declared unmappable, so a request for them is a config error rather than a flag that is silently ignored.
• agent.NewWithOptions is the one funnel: it validates Options.Profile and splices the mapped args after the operator's raw agent_args_override args, so both the pipeline (cfg.AgentProfileFor) and eval replay (Candidate.Profile()) reach every harness by the same path. Never re-derive a model or effort flag at a call site.
• Precedence is fixed: a raw agent_args_override flag that already pins a knob natively wins and the mapped value is not emitted, which is what keeps every pre-agent_config configuration byte-identical and stops a harness receiving one knob twice. agent_config is global-only for the same reason as agent_args_override.
• Eval candidates are agent,model=<model>[,effort=<level>] (the previous agent+model spelling is refused with a migration message), effort is part of the persisted candidate identity, and agentNeutralGlobalConfig strips agent, agent_args_override, and agent_config so a replay never inherits the capturing machine's pins.
• Keep eval replay identity comparison centralized in agentcfg.ServedMatchesRequested; do not compare an adapter's reported model directly at call sites. The user-facing normalization semantics and candidate guidance live in docs/src/content/docs/reference/eval.md.
• Regressions: internal/agentcfg, internal/agent/profile_test.go, internal/config/config_agent_config_test.go, internal/daemon/pipeline_agent_profile_test.go, TestParseCandidate*, TestReplayPinsCandidateModelAndEffortOnTheHarness, TestCaptureStripsEveryHarnessPinFromThePinnedConfig, TestServedMatchesRequested, TestReplayPiModelIdentityComparison.