feat(strategy_ladder_buys): cap consecutive rungs per drawdown

[peeyush20]

Closes a gap surfaced in the README review: ladder_buys has nothing stopping it from buying every drop in a sustained downtrend until cash runs out. Adds a per-stock cap on both rung count and cumulative notional invested, both reset on any sell.

New tunables (persistence/config/strategy_defaults.json)

• ladder_buys.max_rungs = 4 — entry counts as rung 1, so this
  means entry + 3 dip adds before
  the strategy pauses on this stock.
• ladder_buys.max_position_usd = 2000 — additive belt-and-suspenders. Even
  if max_rungs were misconfigured,
  cumulative invested-since-last-sell
  on one stock is bounded.

State (lib/pool.sh)

• New per-stock fields strategy_config.ladder_buys.consecutive_buys and consecutive_invested_usd. Initialised to 0 in pool_add_stock.
• New helper pool_increment_ladder_consecutive bumps both atomically. Uses // 0 fallbacks so it's safe against the pre-counter empty {} layout in existing pool.json files.
• pool_set_last_sell now resets both counters. Triggers on any sell — partial (profit_take) or full (trailing_stop) — so each new accumulation cycle starts with a fresh budget.

Strategy enforcement (.claude/skills/strategy_ladder_buys/scripts/apply.sh)

• Two new short-circuit checks before notional sizing: rung_cap_hit — emits status:"skipped" reason "max_rungs reached (N/M)"
  pos_cap_hit — emits "max_position_usd reached ($X/$Y)"
• The notional sizing min() is extended with remaining_pos = max_pos -
  consecutive_invested, so the final rung uses a partial notional rather
  than overshooting the cap.
• Successful orders' reason now includes "(rung N/M)" — operator can see
  cap pressure at a glance in the snapshot.

Doc (.claude/skills/strategy_ladder_buys/SKILL.md)

• New "Consecutive-rung cap" section explaining both knobs, reset-on-sell semantics, the per-stock override path, and the manual-close caveat (closing a position in the Alpaca web UI does NOT reset the counter — same divergence problem as elsewhere; pointed at so operators hitting it know what to do).
• First-rung handling note updated to flag that cold-start counts as rung 1.

Backward compat

• Existing pool.json entries with ladder_buys: {} still work. The // 0 fallbacks treat missing fields as zero on read; the next pool_increment populates them.
• pool_strategy_config already merges defaults into per-stock overrides, so the new max_rungs / max_position_usd defaults automatically apply to stocks added before this change.

Verified

• bash -n on pool.sh and apply.sh; jq parse on strategy_defaults.json.
• 7-step functional test of pool_add_stock / pool_increment_ladder_consecutive / pool_set_last_sell / pool_strategy_config in a sandbox repo, including a backward-compat path and cap-hit arithmetic. All pass.