feat: add peg, chaser, and trailing-stop order types

[vkbalakrishnan]

What

Adds first-class support for BitMEX pegged, chaser, and trailing-stop orders — on the CLI and as MCP tools.

Why

order buy|sell already forwarded --order-type/--exec-inst as freeform strings, so the labels Pegged/Fixed/ChaserClassic were reachable — but there were no flags for pegPriceType or pegOffsetValue, the two fields every one of these order types requires. As a result none of them could actually be placed via the CLI or its MCP tools.

What changed

Primitive flags (thin pass-through) on buy/sell:

• --peg-price-type <PrimaryPeg|MarketPeg|TrailingStopPeg|LastPeg|MidPricePeg>
• --peg-offset-value <N>
• --peg-offset-value also added to amend (amend a live peg without cancel/replace).

Guided subcommands that set ordType/execInst and validate the offset sign locally:

• order chase <symbol> <side> <qty> --offset N [--bothways] → Pegged + ChaserClassic/ChaserBothways; re-prices to follow the top of book.
• order trailing-stop <symbol> <side> <qty> --offset N [--limit-price P] [--trigger T] → Stop/StopLimit + TrailingStopPeg.

Sign-convention safety

The two guided commands enforce the pegOffsetValue sign client-side, returning a validation error before the order reaches the exchange. The rules are opposite, which is exactly the footgun this guards:

• Chaser: Buy offset ≤ 0, Sell offset ≥ 0 (rests passively behind the touch; max 5/account).
• Trailing stop: Sell offset ≤ 0, Buy offset ≥ 0.

MCP + docs

• agents/tool-catalog.json: peg params on order buy/order sell, peg_offset_value on amend, and new order chase / order trailing-stop entries with oneOf enums. Also fixes pre-existing drift where the order sell entry was missing tif/exec_inst (needed for chaser/pegged via the primitive path over MCP).
• Docs: README.md, CONTEXT.md, AGENTS.md, CLAUDE.md.

Examples

# Pegged limit (requires exec_inst Fixed)
bitmex order buy XBTUSD 100 --order-type Pegged --exec-inst Fixed \
  --peg-price-type PrimaryPeg --peg-offset-value -1

# Chaser
bitmex order chase XBTUSD Buy  100 --offset -1
bitmex order chase XBTUSD Sell 100 --offset  1 --bothways

# Trailing stop
bitmex order trailing-stop XBTUSD Sell 100 --offset -100
bitmex order trailing-stop XBTUSD Sell 100 --offset -100 --limit-price 49000

Testing

• Unit (order.rs): as_api mappings, build_order_body peg fields, chase/trailing sign rules, ordType/execInst selection.
• MCP (registry.rs, server.rs): new tools registered with correct schema; full MCP→CLI arg routing for the side positional + hyphen-value --offset.
• Integration (cli_tests.rs): --help for the new subcommands; --validate dry-runs assert the emitted body and sign rejection.
• cargo test (112 lib + 22 integration) green; cargo clippy clean on order.rs.

Semantics grounded in BitMEX's swagger.json, the engine's NewOrderChaserValidator, and the Chaser order blog post.