feat(desktop): remote agent backend providers in channel Add Agent dialog

[tlongwell-block]

Summary

The channel-context Add Agent dialog (+ button in the channel header) can now deploy agents to remote infrastructure via backend providers.

Previously this dialog only created local agents. The global Create Agent dialog (Agents view) already had full "Run on" support from #134, but the quick-add channel flow did not. This PR threads backend provider selection through the channel dialog using the same hooks and patterns.

Multi-agent support

Multiple agents can be deployed to the same remote target simultaneously. The desktop calls deploy once per agent — each invocation is independent. Backend providers are responsible for multi-agent coexistence (e.g., per-agent config isolation, process management). The GUI does not know or care about the implementation details — it just sends N deploy requests with the same provider_config, and each returns a unique agent_id.

Each agent runs as an independent sprout-acp process with its own Nostr identity. Stop/shutdown targets individual agents by pubkey via !shutdown.

What it looks like

No backend providers on PATH → dialog is identical to today. Zero behavior change.

Backend providers discovered → a "Run on" selector appears above the Runtime dropdown:

Run on
[This computer ▼]        ← default, local mode (unchanged)
[my-provider    ]        ← select to deploy remotely

Selecting a provider shows:

• ⚠️ Trust warning (provider receives the agent's private key)
• Config fields dynamically rendered from the provider's config_schema
• Full batch support — select multiple personas, teams, or Generic just like local mode

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Changes

File	What	ΔLoC
channelAgents.ts	backend field on CreateChannelManagedAgentInput; thread to createManagedAgent with provider-mode flags; spawnError check	+14/−1
ChannelMembersBar.tsx	Wire useBackendProvidersQuery, pass to dialog	+4/−0
AddChannelBotDialog.tsx	"Run on" selector, trust warning, probe effect, provider config fields, batch support in provider mode	+162/−17
ProviderConfigFields.tsx	New — extract shared coerceConfigValues + ProviderConfigFields from CreateAgentDialog	+85/−0
CreateAgentDialog.tsx	Import from shared module (dedup)	+2/−89
check-file-sizes.mjs	Narrowly scoped override for AddChannelBotDialog (600 lines)	+1/−0

Total: +267/−88 across 6 files.

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Design decisions

spawnError check is load-bearing

Tauri's create_managed_agent returns Ok() even on deploy failure — spawnError carries the message. Without the explicit check in createChannelManagedAgent, deploy failures would be silently swallowed and the agent would appear to succeed.

Batch creation works in provider mode

The GUI calls deploy N times with the same provider_config. Each call is an independent invocation of the backend provider binary (stdin/stdout JSON, one-shot). Backend providers are responsible for multi-agent coexistence — the desktop makes no assumptions about how the provider manages multiple agents on the same target.

ACP runtime always required

Unlike CreateAgentDialog (which has Advanced Setup fields that fall back to Rust defaults), the channel dialog has no Advanced Setup. The Runtime dropdown is the only way to specify agentCommand/agentArgs, which flow through the deploy payload to the remote provider.

Deploy-before-attach ordering is safe

The sprout-acp harness subscribes to membership notifications before channel discovery, then calls discover_channels() via REST. So: create → deploy (agent starts, discovers channels) → attach to channel (agent picks up via live notification subscription). No restart required.

Explicit backend: { type: "local" } in local mode

Matches CreateAgentDialog pattern. Rust has Default for BackendKind = Local, so undefined would work, but explicit is safer and consistent.

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Testing

• TypeScript compiles cleanly
• Biome check passes (warnings match reference CreateAgentDialog)
• File size check passes
• Pre-commit hooks pass (biome, clippy, rust tests, desktop build)
• Pre-push hooks pass (all 6 checks)
• Manual testing: 5 agents deployed simultaneously to one remote target via backend provider ✅
• E2E tests deferred (no existing coverage for this dialog's provider mode)