refactor(appkit): separate plugin context binding from plugin construction

[MarioCadenas]

Summary

Splits the Plugin base-class lifecycle into two phases:

• Construction (now pure): new Plugin(config) does nothing side-effectful. Factories like toPlugin can return a PluginData tuple at module scope without pulling in CacheManager, TelemetryManager, or PluginContext.
• attachContext({ context, telemetryConfig }): runs inside createApp after all plugins have been constructed. Binds telemetry, cache, and the PluginContext mediator onto each instance, then fires setup().

This unblocks the new agents() plugin (PR #294) which needs to construct agent definitions at module scope and resolve their plugin references lazily at setup() time, and it's a prerequisite for any factory that wants to hand back a typed, reusable plugin handle before createApp runs.

Notes

• ServerPlugin.setup() now defers its HTTP start to the setup:complete lifecycle hook so that routes registered by deferred-phase plugins (like agents()) are visible when the server binds. Without this, extendRoutes() would run before AgentsPlugin.injectRoutes() had a chance to add its handlers.
• Adds attachContext to BasePlugin in packages/shared/src/plugin.ts.

PR Stack

1. Shared types + Adapters — feat(appkit): add shared agent types and LLM adapter implementations #282
2. Tool types + MCP client — feat(appkit): add FunctionTool, HostedTool types and MCP client #283
3. Agent plugin core + ToolProvider implementations — feat(appkit): add agent plugin core and ToolProvider implementations #284
4. PluginContext mediator — feat(appkit): add PluginContext mediator for inter-plugin communication #285
5. createAgent + agent-app + docs (v1) — feat(appkit): add createAgent wrapper, agent-app, and API docs #286
6. Plugin context-binding separation (this PR)
7. agents() plugin + createAgent(def) + .toolkit() — feat(appkit): add agents() plugin, createAgent() factory, and .toolkit() #294
8. agent-app + docs migrated to agents() — feat(appkit): migrate agent-app and docs to the new agents() plugin #295
9. Relocate shared agent utilities — refactor(appkit): relocate shared agent utilities into plugins/agents #296
10. preparePlugins forwards eager instance — refactor(appkit): forward eager plugin instance through preparePlugins #297
11. fromPlugin() API — feat(appkit): add fromPlugin() for referencing plugin tools in code-defined agents #298
12. Retire deprecated agent() + createAgentApp — chore(appkit): remove deprecated agent() plugin and createAgentApp shortcut #299
13. Retire toPluginWithInstance + bug fixes — refactor(appkit): retire toPluginWithInstance; consolidate on fromPlugin + fix schema/routing bugs #300

Test plan

• Full vitest suite (unchanged test count, all passing)
• Typecheck clean across all 8 workspace packages

[MarioCadenas]

Superseded by the v2 6-PR stack:

1. Shared agent types + LLM adapters — feat(appkit): shared agent types and LLM adapter implementations #301
2. Tool primitives + ToolProvider surfaces — feat(appkit): tool primitives and ToolProvider surfaces on core plugins #302
3. Plugin infrastructure (attachContext + PluginContext) — feat(appkit): plugin infrastructure — attachContext + PluginContext mediator #303
4. agents() plugin + createAgent(def) + markdown-driven agents — feat(appkit): agents() plugin, createAgent(def), and markdown-driven agents #304
5. fromPlugin() DX + runAgent plugins arg + toolkit-resolver — feat(appkit): fromPlugin() DX, runAgent plugins arg, shared toolkit-resolver #305
6. Reference app + dev-playground + docs — feat(appkit): reference agent-app, dev-playground chat UI, docs, and template #306

The v2 stack reorganizes the same work so no PR ships API that a later PR deletes. Start at #301 for the new entry point. Branches from this older stack are preserved unchanged.