docs: refresh SnapTrade operational runbooks

[cursor]

Summary

• Update the portfolio-sync runbook from CSV-first ingestion to the verified SnapTrade routing and approval workflow.
• Update the margin-dashboard runbook for live metrics, implemented status bands, strategy gates, and explicit CSV fallback.
• Refresh runbook navigation, session-context wording, skill inventory, and role-based ownership references.
• Why: the indexed runbooks still described retired CSV-primary position and balance workflows after the SnapTrade cutover.
• Link to related issue(s): none

Codepaths covered

• src/integrations/snaptrade/cli.py, models.py, and client.py
• src/analysis/margin_metrics.py
• PortfolioSyncing, margin-management, dividend-tracking, and formula-protection workflows
• .claude/hooks/load-fin-core-config.ts session-start behavior

Key knowledge gaps addressed

• Credential and account-routing prerequisites, fail-closed refusal behavior, and read-only CLI boundaries
• Position/options/cash/margin mappings and approval requirements before Google Sheets writes
• Margin metric source semantics, null SnapTrade fields, first-account selection, threshold distinctions, and CSV fallback commands
• Troubleshooting for missing credentials/config, refused accounts, unavailable equity, and protected formulas

Test Plan

• Local links resolve in all four changed documentation files
• git diff --check origin/main...HEAD
• Compliance scan passes for the pushed commits
• Gitignore protection integration test passes
• Source-backed accuracy audit passes against implementation, tests, and operational skills
• Full pytest suite — not run because this Cloud image did not contain uv, pytest, or project Python dependencies; no code changed
• Ruff — not applicable; no Python changed
• Mypy — not applicable; no Python changed

Review Checklist

• No secrets or PII committed
• Documentation updated
• Test coverage maintained; documentation-only change