Agent manifest parameters: launchpad UI, parameter substitution, session persistence #72

Summary

Agent manifests should be able to declare typed parameters that callers supply at session-creation time. A first-class UI collects those values before creating the session, and the agent-session entity persists them so sessions can be reconstructed correctly after restart.

Motivation

Manifests targeting github-copilot need a working-directory per invocation. In future, every manifest will accept a trust-profile parameter. These values cannot be baked into the manifest file.

Design

See docs/design/agent-manifest-parameters.md for the full design.

Scope

Backend (Phantom.Workspaces.Llm.Core)

• AgentDefinitionParameterSubstitutor — validates supplied values against AgentManifest.Parameters, applies defaults, substitutes ${name} placeholders in model.options string values.
• CreateAgentChatRequest.ModelParameters — new IReadOnlyDictionary<string, string>? property; AgentFactory calls the substitutor before CreateChatClient.

Entity schema

• agent-session.json: add parameter-values (object, string-valued); rename agent-definition-entity-id → agent-source-entity-id.

UI (Phantom.Workspaces)

• AgentManifestLaunchpadViewModel — shown when opening an agent-manifest or agent-definition entity; parameter-entry form with Start Session and Edit Manifest buttons.
• AgentManifestParameterRowViewModel — one row per declared parameter; bindable Value; directory-picker affordance for working-directory-style parameters.
• AgentManifestEditorViewModel — manifest JSON editor; exposes Clone, Save, and Start Session (save-then-open-launchpad) commands; Clone is prominently surfaced.
• OpenAgentManifestShortcutHandler / OpenAgentDefinitionShortcutHandler — replaced to open the Launchpad tab instead of directly starting a session. If the manifest declares no parameters the Launchpad auto-starts immediately (zero-friction path).
• EditAgentManifestShortcutHandler — new; handles Shortcut.Edit on agent-manifest.
• OpenAgentSessionShortcutHandler — reads parameter-values from the session entity on resume.
• Views: AgentManifestLaunchpadView.axaml, AgentManifestEditorView.axaml.

Tests

• AgentDefinitionParameterSubstitutorTests — required param missing, default applied, placeholder substitution, unknown param ignored.

Parameters of note

• working-directory (Copilot) — Launchpad row shows a directory-picker button.
• trust-profile (future, all manifests) — Launchpad row will be a dropdown bound to available llm-trust-profile entities. Out of scope here; mechanism is intentionally general.

Parameter typing

AgentManifest.Parameters (PropertySchema from AgentSchema) already models parameters with Name, Kind, Description, Required, Default. Deeper type-aware UI (e.g. integer spinners, boolean toggles) is deferred; all values are collected as strings in this iteration.