Add `filter` param to `get_positions` (losers_only, winners_only, all)

[autojack-bot]

Problem

get_positions returns all ~40 positions every time, which eats significant context window in LLM workflows — even when you only care about a subset.

Proposed Solution

Add an optional filter query param to get_positions:

• losers_only — return only positions with negative P&L (tax-loss harvesting, "what's on sale")
• winners_only — return only positions with positive P&L
• all — current behavior (default, backwards compatible)

Example Usage

GET /positions?filter=losers_only

Why

For common portfolio analysis tasks (tax-loss harvesting, reviewing underperformers), this cuts the response from ~40 positions to ~10-12 — a ~70% reduction in context usage with no loss of relevant data.

Notes

• Default should be all to maintain backwards compatibility
• Useful companion to the field-slimming issue (chore: switch to release-please for automated versioning #2)