Implement Phase 2 portfolio construction and Phase 3 research integration

[Claude]

Implements portfolio rebalancing pipeline and Content API research feeds as specified in the five-layer harness architecture (spec.md Phase 2 & 3).

Portfolio Construction (internal/portfolio/)

Core logic (463 LOC):

• SyncCurrent() - Computes current portfolio from account state + quotes with position weights
• ComputeDiff() - Calculates target vs current delta, generates rebalance actions with quantity estimates
• ProcessRebalance() - Converts portfolio/rebalance/pending.json to ORDER entries in beancount ledger
• ValidateTarget() - Ensures target weights sum to 1.0, validates allocations

Data flow:

portfolio/target.json (user/agent writes)
  → portfolio/current.json (controller generates)
  → portfolio/diff.json (controller computes delta)
  → portfolio/rebalance/pending.json (user/agent approves)
  → trade/beancount.txt ORDER entries (controller executes)
  → portfolio/history/{timestamp}.json (controller archives)

Threshold-based rebalancing ignores adjustments <1% or $100 to reduce churn.

Research Feeds (internal/research/)

Content API integration (267 LOC):

• RefreshFeeds() - Reads research/watchlist.json, fetches news/topics via Content API
• GenerateSummary() - Scans feeds and quotes, produces research/summary.json availability index

Writes structured feeds to:

• research/feeds/news/{SYMBOL}/latest.json
• research/feeds/topics/{SYMBOL}/latest.json

Controller Integration

Added to polling loop (cmd/longbridge-fs/main.go):

1. Portfolio sync → diff → rebalance processing (after account refresh)
2. Research refresh (after quote updates, skipped in mock mode)

All operations are non-blocking - failures logged but don't halt the controller.

Type Definitions

Extended internal/model/types.go with:

• TargetPortfolio, CurrentPortfolio, PortfolioDiff, Adjustment
• RebalancePending, RebalanceOrder
• Watchlist, NewsFeed, TopicsFeed, ResearchSummary

Maps directly to file structures defined in spec.md sections 6 & 4.

Deferred

--auto-rebalance mode intentionally omitted - manual approval via pending.json is safer for production. Current workflow requires explicit user/agent confirmation before ORDER generation.