[ISSUE-480] chore: remove unreferenced dead-code files in packages/server

[1996fanrui]

Summary

Remove 7 source files from packages/server that have zero inbound references anywhere in the repo. Identified via Knip dead-code analysis (npx knip + npx knip --production) and cross-verified with manual grep across .ts/.tsx/.mjs/.js, package.json, vitest config, and docs.

Files removed

File	Verification
packages/server/src/server/agent/llm-openai.ts	Zero references in repo
packages/server/src/server/agent/orchestrator.ts	Zero references (orchestrator-instructions.ts is a distinct file sharing only the name prefix)
packages/server/src/server/agent/model-resolver.ts	Only used by its own model-resolver.test.ts; both removed together
packages/server/src/server/agent/model-resolver.test.ts	Companion test of the removed module
packages/server/src/server/daemon-e2e/checkout-debug.ts	Zero references in repo
packages/server/src/poc-commands/investigate-command-output.ts	Zero references — leftover POC
packages/server/src/poc-commands/run-poc.ts	Zero references — leftover POC (header literally says "POC Script")

Total: 7 files, 711 lines deleted.

Scope

• Only removes files whose absence of references has been verified with both Knip and manual grep.
• Does NOT touch other Knip findings (unused dependencies, unused exports, scripts that may be invoked externally) — those need more per-item review.
• Does NOT add any Knip CI gate, because a clean CI integration would need substantial entry-point configuration to avoid false positives from dynamically-loaded providers, spawned scripts, and the public @getpaseo/server SDK surface.

Test plan

• tsc --noEmit on packages/server passes with no new errors (the only pre-existing error is @getpaseo/highlight not being built in the local workspace, unrelated to this change).
• CI typecheck passes
• CI test suite passes

Closes #480