feat: add close-positions CLI command and replace_symbol projection mode (#65 #66)

Part 1 — close-positions CLI (#65):
- Add `close-positions` subcommand delegating to `close_open_positions()`.
- Accepts repeated `--symbol` and `--ticket` filters (AND semantics).
- Supports `--dry-run` (no `--yes` required); live execution requires `--yes`.
- Fails closed with `BadParameter` when neither `--symbol` nor `--ticket` is given.
- Exports normalized `OrderExecutionResult` list as a DataFrame (request/response
  serialized as JSON strings for clean CSV/JSON/Parquet/SQLite output).
- `order-send` remains the raw expert path; `close-positions` is the safer
  high-level helper that builds correct close requests automatically.

Part 2 — ProjectionMode and replace_symbol (#66):
- Add `ProjectionMode = Literal["add", "replace_symbol"]` type alias.
- Add optional `projection_mode` parameter to `calculate_symbol_group_margin_ratio`.
  Default `"add"` preserves existing additive behavior.
  `"replace_symbol"` subtracts current margin for `new_symbol`, then adds
  candidate margin — the subtraction and addition are atomic (suppressed together).
- Export `ProjectionMode` from `mt5cli` and add to `STABLE_SDK_EXPORTS`.
- No mteor-specific strategy, risk-threshold, or policy logic added.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -178,10 +178,13 @@ python -m mt5cli -o account.csv account-info
 | `recent-history-deals` | Export historical deals from a recent trailing window                                                        |
 | `mt5-summary`          | Export terminal/account status summary                                                                       |
 | `order-check`          | Check funds sufficiency for a trade request                                                                  |
-| `order-send`           | Send a trade request to the trade server (`--yes` required)                                                  |
+| `order-send`           | Send a raw trade request to the trade server (`--yes` required; expert path)                                 |
+| `close-positions`      | Close open positions by `--symbol` or `--ticket` (`--yes` required for live; `--dry-run` available)          |
 | `collect-history`      | Bundle rates, ticks, history-orders, and history-deals for one or more symbols into a single SQLite database |
 
 Use `order-check` to validate a request payload before running `order-send --yes`.
+`close-positions` is the safer high-level alternative that builds correct close
+requests automatically. At least one `--symbol` or `--ticket` must be provided.
 
 ### `collect-history`