Align agent mode harness with .NET

[Michelle Clayton (michelle-clayton-work)]

Add configurable mode tools, notification suppression, named tool arguments, and matching empty-mode validation.

• Complete .NET agent/harness port #1141

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved issues block approval.

Review effort: Lite
Findings: None

What changed in this PR

Aligns the Go agent-mode harness with the .NET implementation.

Changes:

• Adds configurable mode-tool suppression.
• Supports named mode_set arguments.
• Adds notification suppression.
• Validates empty mode configurations and expands tests.

File	Summary
agent/​harness/​agentmode/​agentmode.go	Implements configuration, tool, instruction, and notification behavior.
agent/​harness/​agentmode/​agentmode_test.go	Tests validation, named arguments, disabled tools, and notifications.

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[github-actions]

API consistency review

Result	Scope
aligned	public API / user-visible behavior

Changed Go contract

• agentmode.Config.Modes: semantics changed so nil uses the default plan/execute modes, while a non-nil empty slice now panics ("at least one mode must be configured") instead of silently falling back to defaults.
• mode_set tool: input schema changed from a generic single positional argument (Arg0) to a named JSON field mode ({"mode": "..."}).
• Provider.SetModeForSession(session, mode, disableNotification ...bool) error: added an optional variadic disableNotification argument (errors if more than one value is supplied), matching the notification-suppression behavior already present in SetModeForSessionSilently.

Upstream evidence reviewed

Implementation	Source	Contract checked
.NET	AgentModeProviderOptions.cs — Modes property doc: "When null (the default)... uses two built-in modes"	Nullable Modes defaults when unset
.NET	AgentModeProvider.cs constructor: if (this._modes.Count == 0) throw new ArgumentException("At least one mode must be configured.", ...)	Empty (non-null) mode list is invalid, not treated as "use defaults"
.NET	Same file, CreateTools: AIFunctionFactory.Create(async (string mode) => ..., new AIFunctionFactoryOptions { Name = "mode_set", ... })	mode_set tool exposes a single named parameter mode, not a positional/generic arg
.NET	Same file: SetModeAsync(AgentSession, string, bool disableNotification, CancellationToken) overload alongside the 2-arg overload	Optional disableNotification parameter with equivalent state-clearing semantics

Assessment

The Go changes bring agentmode in line with the .NET AgentModeProvider: the nil-vs-empty Modes distinction, the named mode tool argument, and the optional notification-suppression parameter on SetModeForSession all match documented .NET behavior and defaults. The Go variadic-bool pattern for the optional parameter is an idiomatic (not semantic) difference from the .NET method overload. No parity issues found.

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