codedb_read: ranged output is line-number-prefixed (not byte-exact) — add a raw read mode #632

Problem

codedb_read with a line range returns line-number-prefixed output — handleRead calls extractLines(content, start, end, /*line_numbers=*/true, …) (hardcoded) and also always prepends hash:<x>\n. So a ranged read is not a byte-exact copy of the source.

Consequence on the agent read path: an agent that located code via codedb_symbol/codedb_search cannot use codedb_read's ranged output as the old_string for an exact-match editor — the 234 | prefixes won't match. So it falls back to a native file read for the pre-edit span, and codedb only ever serves locate, never read+edit. This is the codedb half of justrach/codegraff#66 (graff routes raw reads to its native tool for exactly this reason).

Failing test (src/test_mcp.zig, fails on main)

test "issue-632: codedb_read raw mode returns byte-exact range without line-number prefixes" {
    // ... tmpdir with "alpha\nbeta\ngamma\n", BenchContext, Explorer ...
    // args: {"path":"small.txt","line_start":1,"line_end":2,"raw":true}
    bench_ctx.runDispatch(io, alloc, .codedb_read, &args, &out, &store, &explorer, &agents);
    try testing.expect(std.mem.indexOf(u8, out.items, "alpha\nbeta") != null); // exact bytes
    try testing.expect(std.mem.indexOf(u8, out.items, " | ") == null);        // no line-num prefix
}

Current behavior: ranged output is 1 | alpha\n 2 | beta, so " | " is present and the second assertion fails (error.TestUnexpectedResult). raw:true is silently ignored.

Expected

A raw/byte-exact mode (e.g. raw: true) returns the exact source lines for the range with no line-number prefix (and the hash line suppressed or kept clearly separable), so codedb_read can feed an exact-string edit and serve the full read→edit loop, not just locate.

Fix (sketch)

• Read a raw (or number) bool arg in handleRead; pass !raw as extractLines's line_numbers.
• In raw mode, omit the hash: header (or move it out-of-band) so the body is verbatim.
• Keep the default (prefixed) unchanged for human/inspection use.