Add `--transport` flag to support SSE in addition to stdio

[Bortlesboat]

What

The CLI currently only supports stdio transport (the default for MCP). Add a --transport flag that lets users choose between stdio (default) and sse (Server-Sent Events over HTTP).

Why

SSE transport is useful for:

• Web-based MCP clients that cannot use stdio
• Running bitcoin-mcp as a persistent HTTP service behind a reverse proxy
• Development/debugging (you can curl the SSE endpoint directly)

The mcp library already supports SSE — this is just wiring up the CLI flag.

Where to look

• src/bitcoin_mcp/server.py — the main() function at the bottom (line ~1864)
• Currently it just calls mcp.run() which defaults to stdio
• The FastMCP run() method accepts a transport parameter: mcp.run(transport="sse")

Implementation

parser.add_argument(
    "--transport",
    choices=["stdio", "sse"],
    default="stdio",
    help="MCP transport type (default: stdio)",
)
args = parser.parse_args()
mcp.run(transport=args.transport)

Acceptance criteria

• bitcoin-mcp --transport stdio works (current default behavior)
• bitcoin-mcp --transport sse starts an SSE server
• bitcoin-mcp with no flag defaults to stdio (no breaking change)
• Add a test in the existing TestCLI class that verifies --transport is accepted
• Update README "Configuration" section to mention the flag