Make server port configurable

[StatPan]

Goal

Honor PORT for the Hono server without breaking Vite proxy, Docker, or CLI expectations.

Derived from #14 Phase 6 dirty branch handoff.

Scope

• Keep the runtime change focused on src/server/index.ts.
• Update docs/config references if they assume a hardcoded port.
• Confirm compose/dev defaults remain 3001.

Out of Scope

• Do not change OpenCode approval/question behavior.
• Do not change deployment topology or auth behavior.

Acceptance Criteria

Functional:

• Server uses PORT when set.
• Server defaults to 3001 when PORT is unset.
• README/compose/dev guidance remains accurate.

Code/product quality:

• Port parsing is explicit and handles invalid values predictably.
• The change is covered by a smoke test or focused config/unit test.

Verification

• PORT=3002 ./node_modules/.bin/tsx src/server/index.ts smoke test if OpenCode/env is available, or a focused config/unit test if not.
• ./node_modules/.bin/tsc -p tsconfig.json --noEmit.
• git diff --check.

Expected Delivery

• Delivery mode: small focused PR.
• Review mode: verify default compatibility and configured-port smoke evidence.
• Merge mode: safe to merge without owner escalation if tests and docs pass.