feat: Phase 2 — L3 Portfolio Construction (v0.4.0)

[Copilot]

Implements the L3 Portfolio Construction layer of the Harness five-layer pipeline, enabling signal-driven rebalance from target weights through to beancount ORDER entries — all file-system driven.

New: internal/portfolio/ package

• target.go — ParseTarget / ValidateTarget: reads portfolio/target.json, validates weights sum ≈ 1.0 and total_capital_pct ∈ (0, 1]
• diff.go — SyncCurrent: builds portfolio/current.json from account/state.json + quote/hold/*/overview.json; ComputeDiff: produces portfolio/diff.json with per-symbol ADD/REDUCE/HOLD/REMOVE actions and estimated delta quantities
• rebalance.go — ExecuteRebalance: converts portfolio/rebalance/pending.json to beancount ORDER entries tagged with source: rebalance + rebalance_id, archives the file to portfolio/history/, then removes it
• portfolio.go — SyncPortfolio orchestrates the three steps above in sequence

Model types (internal/model/types.go)

Added: PortfolioTarget, TargetPosition, PortfolioCurrent, CurrentPosition, PortfolioDiff, PortfolioAdjustment, RebalancePending, RebalanceOrder

Controller integration (cmd/longbridge-fs/main.go)

• portfolio.SyncPortfolio(root, autoRebalance) injected into polling loop after account refresh
• New --auto-rebalance flag (default false) — when false, only manually placed pending.json files are executed; the diff is always computed for Agent review
• fs init now creates a default portfolio/target.json skeleton

Example rebalance flow:

# Agent writes portfolio/target.json with desired weights
# Controller next tick:
#   1. Writes portfolio/current.json  (live snapshot)
#   2. Writes portfolio/diff.json     (AAPL: REDUCE -67, 700.HK: ADD +80)
# Agent reviews diff, writes portfolio/rebalance/pending.json
# Controller next tick:
#   3. Appends to trade/beancount.txt:
#        ; source: rebalance
#        ; rebalance_id: rebal-20260330-001
#   4. Archives pending.json → portfolio/history/