feat: change default port from 6969 to 3207 with fallback chain

[BYK]

Summary

• Replace the hardcoded default port 6969 with a fallback chain: 3207 (primary) -> 5673 (fallback) -> random OS-assigned port
• Add port file discovery (~/.local/share/lore/gateway.port) so plugins find the gateway even on a random fallback port
• Refactor both OpenCode and Pi plugins to use the gateway's startGateway() instead of duplicating startup logic

Port Mnemonics

• 3207: flip upside-down (calculator-word style) -> 7=L, 0=O, 2=R, 3=E -> LORE
• 5673: T9 phone keypad -> 5=L, 6=O, 7=R, 3=E -> LORE

Details

Gateway core

• config.ts: Added DEFAULT_PORTS = [3207, 5673], DEFAULT_PORT, and portExplicit flag to GatewayConfig
• portfile.ts (new): writePortFile(), readPortFile(), removePortFile() for plugin discovery
• server.ts: Fixed port 0 + multi-host binding — pins resolved port after first Bun.serve() so all hosts share the same actual port
• start.ts: Rewrote startGateway() with fallback loop. Explicit --port / LORE_LISTEN_PORT skips the chain.
• index.ts: Exported startGateway, probeGateway, readPortFile, DEFAULT_PORTS, DEFAULT_PORT

Plugins

• OpenCode + Pi: Replaced hardcoded http://127.0.0.1:6969 with resolveGatewayUrl() (probes port file + known defaults). Refactored startInProcess() to delegate to startGateway(), eliminating duplicated EADDRINUSE handling.

Docs

• Updated all 11 references to 6969 across help text, CLI comments, README, and landing page

Verification

• bun run typecheck — all 4 packages pass
• bun test — 1,304 tests pass, 0 fail (core: 772, gateway: 523, opencode: 9)
• bun run build — all packages build successfully
• Zero remaining references to 6969 in source/docs