chore(appkit): remove deprecated agent() plugin and createAgentApp shortcut

[MarioCadenas]

Summary

Pure dead-code removal. With agents() + createAgent(def) + fromPlugin now covering every use case previously served by the transitional agent() plugin and createAgentApp shortcut, the deprecated surface is removed entirely.

Removed

• packages/appkit/src/plugins/agent/ — entire folder (agent.ts, config-loader.ts, types.ts, manifest.json, tests/)
• packages/appkit/src/core/create-agent.ts — the old createAgent app-shortcut. Superseded by packages/appkit/src/core/create-agent-def.ts (pure factory) combined with agents()
• packages/appkit/src/core/tests/create-agent.test.ts — tests for the removed shortcut
• docs/docs/api/appkit/Function.createAgentApp.md — stale generated API doc
• agent and createAgentApp exports from packages/appkit/src/index.ts, and the ./agent re-export from packages/appkit/src/plugins/index.ts

Docs

The migration guide's "Gradual migration" and "Removal timeline" sections are dropped — they described a coexistence window that no longer exists. The "Name changes at a glance" table now points at the final shape instead of the deprecated alias.

Net change

−1,761 lines. No runtime behavior change for anyone on the new API. Anyone still importing agent or createAgentApp will see a clear module-not-found error pointing at the new exports.

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 — refactor(appkit): separate plugin context binding from plugin construction #293
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 (this PR)
13. Retire toPluginWithInstance + bug fixes — refactor(appkit): retire toPluginWithInstance; consolidate on fromPlugin + fix schema/routing bugs #300

Test plan

• Full vitest suite passing (1,287 tests, down from 1,317 — delta is tests for deleted deprecated code)
• Typecheck clean
• Build clean

[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.