Implement --auto-rebalance mode, signal/definitions/ declarative config, and built-in indicators

[Copilot]

Advances Phase 2 (v0.4.0) and Phase 3 (v0.5.0) of the Harness pipeline: auto-rebalance closes the portfolio drift → order loop without manual intervention; the signal layer adds declarative config + built-in indicator computation.

--auto-rebalance mode (Phase 2)

New AutoCreatePending() in internal/portfolio/rebalance.go reads portfolio/diff.json and auto-generates portfolio/rebalance/pending.json when requires_rebalance: true. Guards against double-execution (no-op if pending.json already exists). Wired into the controller loop after ComputeDiff().

longbridge-fs controller --root ./fs --auto-rebalance

Signal definitions + built-in indicators (Phase 3)

New internal/signal/ package:

• definitions.go — ListDefinitions() scans signal/definitions/*.json, filters enabled: true; validates type as builtin or external
• indicator.go — Three built-in indicators reading close prices from quote/hold/{SYMBOL}/D.json:
  • SMA_CROSS — fast/slow SMA crossover with relative-distance strength
  • RSI — Wilder RSI with configurable overbought/oversold thresholds
  • PRICE_CHANGE — percentage change over a configurable window
• signal.go — ComputeAll() orchestrates definition scan → kline load → indicator dispatch → per-symbol output/latest.json + history.jsonl → signal/active.json aggregation
• output.go — WriteOutput, AppendHistory, WriteActiveSignals

Example signal definition (signal/definitions/sma_crossover.json):

{
  "name": "sma_crossover",
  "type": "builtin",
  "version": 1,
  "params": { "indicator": "SMA_CROSS", "fast_period": 5, "slow_period": 20 },
  "symbols": ["AAPL.US"],
  "enabled": true
}

ComputeAll() is called each controller cycle (after research refresh, before portfolio sync) via signalpkg alias to avoid collision with os/signal. Also completes the signal/active.json aggregation item from Phase 3.

New model types added: SignalDefinition, SignalOutput, SignalEntry, ActiveSignals, ActiveSignalEntry.