feat: add Hedge Mode (MultiWay position mode) support

[vkbalakrishnan]

What

Adds Hedge Mode (MultiWay position mode) to the CLI, letting users hold independent Long and Short positions on the same contract instead of netting into one.

Switch position mode — POST /user/positionMode

bitmex account position-mode multiway   # enable Hedge Mode (alias: hedge)
bitmex account position-mode oneway      # back to One-Way (netting)
bitmex position mode multiway            # alias for `account position-mode`

account position-mode is the canonical command (sibling of account margining-mode, which posts to the analogous /user/marginingMode); position mode is a CLI alias delegating to the same handler. Optional --target-account-id for paired/sub-accounts. Confirmation-gated like other state-changing commands (skip with --yes). MultiWay sends {"positionMode":"MultiWay"}; One-Way omits the field, per the API contract.

Hedge legs on orders — strategy field on POST /order

bitmex order buy  XBTUSD 100 --price 50000 --strategy Long
bitmex order sell XBTUSD 100 --price 52000 --strategy Short

--strategy accepts oneway / long / short and maps to the exact API casing. The strategy column is now shown in position list and order list.

MCP

agents/tool-catalog.json gains the strategy param on buy/sell and a new dangerous account position-mode tool. The position mode alias is intentionally CLI-only (no catalog entry) to avoid a duplicate MCP tool.

Docs

README / AGENTS / CONTEXT document Hedge Mode, with links to the BitMEX support articles.

Testing

• cargo build and cargo test pass — 98 unit + 16 integration tests (new coverage: position-mode body builder, order strategy builder, MCP registry, and help-text assertions).
• cargo clippy is clean on the changed files.
• Smoke-tested the built binary: --validate shows "strategy":"Long" in the order body (omitted when unset); all mode aliases (multiway/hedge/oneway/multi-way/one-way) route to /user/positionMode; invalid values are rejected by clap.

Notes

• Scoped change, no unrelated refactoring or formatting.
• For One-Way the positionMode field is omitted (swagger: "leave empty for one way mode"); equivalent to sending null.

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