Phase 7: finish manifest channel migration

[sandl99]

Part of #3896.

Summary

Finish the messaging channel manifest migration by routing the remaining channel-specific lifecycle behavior through manifests, hooks, and shared helpers under src/lib/messaging.

This phase removes the last core assumptions about concrete Telegram, WeChat, Slack, Discord, and WhatsApp implementations so onboarding, diagnostics, policy planning, credential setup, rebuilds, snapshots, and runtime setup consume channel registry metadata and manifest-backed plans.

Scope

• Add and complete manifest/applier hook phase coverage, channel metadata, and status helpers under src/lib/messaging.
• Route health/status, policy preset, credential, rebuild, snapshot, diagnostics, and runtime setup call sites through manifest-backed channel registries.
• Move Slack socket-mode gateway conflict/status handling and runtime deny-feedback behavior into Slack-owned hooks/manifests.
• Move Telegram and Discord bridge health/status behavior into channel-owned hooks and manifest metadata.
• Replace remaining WeChat-specific onboard/config plumbing with manifest-backed messaging config and shared validation paths.
• Keep runtime preload source ownership in TypeScript channel runtime files and package the compiled preload JavaScript through the sandbox Docker build path.
• Expand tests for hook phases, manifest metadata, plan validation, channel status, startup/runtime envs, runtime preload packaging, and CLI fixture stability.
• Preserve intentional compatibility shims and migrations needed for existing sandbox registry/session state.

Acceptance Criteria

• PR refactor(messaging): finish manifest channel migration #5338 completes this phase's implementation scope.
• Core messaging flows resolve channel behavior from src/lib/messaging manifests, hooks, and shared helpers instead of hard-coded channel switch maps.
• Runtime preload JavaScript is generated from TypeScript sources through the targeted preload compile path and is not maintained as separate runtime source.
• Existing sandbox registry/session data remains compatible.
• Targeted messaging tests, source-shape checks, type checks, hooks, and CI pass.
• No user-facing channel behavior changes are introduced beyond manifest-backed diagnostics/status fidelity.