Add console Minesweeper with a UI-free engine

Rules and board state live in Minesweeper.Core with no console calls, so the same board drives both the CLI and the tests.

Engine: seeded or random boards with classic presets; first click always safe (and its ring too when the board has room); cascading reveal that stops at numbers; flagging and chording; win by clearing every safe cell; FromMines() for deterministic layouts.

CLI: typed commands ("3 4", "f 2 5", "chord 6 7") plus optional arrow-key mode. Rows numbered, columns lettered, player input 1-based while engine coordinates stay 0-based, with CommandParser the only place that converts. ANSI colour when writing to a terminal, plain when redirected or NO_COLOR is set.

Tests: 186 covering flood-fill bounds, first-click safety, chording, win/loss, command parsing, session behaviour, and renderer alignment with colour on and off.

Co-authored-by: openhands <openhands@all-hands.dev>

README.md

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+# Minesweeper
+
+A console Minesweeper for the terminal, built on .NET 10. The rules live in a UI-free engine, so
+the same board can be driven by the console client or by tests.
+
+```
+9x9  mines left 10  flags 0/10  time 12s
+Reveal every cell that is not a mine.
+
+     A  B  C  D  E  F  G  H  I
+  1  #  #  1  ·  ·  1  #  #  #
+  2  1  1  1  ·  ·  1  1  2  1
+  3  ·  ·  ·  ·  ·  ·  ·  ·  ·
+  4  2  2  1  ·  ·  ·  ·  ·  ·
+  5  #  #  1  ·  ·  ·  ·  ·  ·
+  6  #  #  2  1  2  1  1  ·  ·
+  7  #  #  #  #  #  #  2  2  2
+  8  #  #  #  #  #  #  #  #  #
+  9  #  #  #  #  #  #  #  #  #
+
+type "<row> <col>" to reveal, f <row> <col> to flag, ? for help, q to quit
+>
+```
+
+## Play it
+
+```bash
+dotnet run --project src/Minesweeper.Cli
+```
+
+| Option | Meaning |
+| --- | --- |
+| `-d, --difficulty <beginner\|intermediate\|expert>` | board preset (default: beginner) |
+| `-b`, `-i`, `-e` | shorthand for the three presets |
+| `-k, --keys` | arrow-key play instead of typed commands |
+| `--seed <n>` | deterministic mines, for reproducible games |
+| `-h, --help` | usage, command list, and legend |
+
+Beginner is 9x9 with 10 mines, intermediate 16x16 with 40, expert 30x16 with 99.
+
+## Commands
+
+Rows are numbered `1..N` down the left, columns are lettered `A..Z` (then `AA..`) across the top.
+
+| Command | Does |
+| --- | --- |
+| `<row> <col>` or `reveal <row> <col>` | open a cell (`r`, `open` are aliases) |
+| `flag <row> <col>` | place or remove a flag (`f`, `mark`) |
+| `chord <row> <col>` | open the neighbours of a number once its flags match (`c`) |
+| `go <row> <col>` | move the cursor without opening anything (`goto`) |
+| `difficulty <name>` | switch preset, by name or `1`/`2`/`3` |
+| `new` | restart at the current difficulty |
+| `help` / `?` | list the commands |
+| `quit` / `q` | leave |
+
+In `--keys` mode: arrows move, `Enter`/`Space` reveals, `F` flags, `C` chords, `1`/`2`/`3` change
+difficulty, `N` starts a new board, `Q` or `Esc` quits.
+
+Colours are on when writing to a terminal and off when the output is redirected; set `NO_COLOR`
+to force them off. The process exits `0` after a win, `1` otherwise.
+
+## Rules
+
+- The first click is always safe. On boards with room, the eight cells around it are safe too.
+- Opening a cell with no adjacent mines cascades outward until it hits numbers.
+- Chording a number whose surrounding flag count matches opens the rest of its neighbours — so a
+  mistaken flag costs you the game.
+- You win by opening every cell that is not a mine; remaining mines are flagged for you.
+
+## Layout
+
+```
+src/Minesweeper.Core   rules and board state, no console calls
+src/Minesweeper.Cli    command parsing, ANSI rendering, the game loop
+tests/                 xUnit tests for the engine, parser, session, and renderer
+```
+
+```bash
+dotnet build   # compiles all three projects
+dotnet test    # runs the suite
+```