mcp: indexing temporary roots causes pathological memory growth

[justrach]

Problem
Indexing a temporary root like /private/tmp succeeds today. In practice that lets MCP cache and reload broad temp directories containing many unrelated worktrees, which can drive codedb --mcp into pathological multi-GB memory usage and swap pressure.

Failing Test

test "issue-77: mcp index accepts temporary-directory roots that cause pathological cache growth" {
    var tmp_name_buf: [128]u8 = undefined;
    const tmp_name = try std.fmt.bufPrint(&tmp_name_buf, "codedb-issue-77-{d}", .{std.time.microTimestamp()});
    const tmp_root = try std.fs.path.join(testing.allocator, &.{ "/private/tmp", tmp_name });
    defer testing.allocator.free(tmp_root);

    std.fs.cwd().makePath(tmp_root) catch |err| switch (err) {
        error.PathAlreadyExists => {},
        else => return err,
    };
    defer std.fs.cwd().deleteTree(tmp_root) catch {};

    const source_path = try std.fs.path.join(testing.allocator, &.{ tmp_root, "sample.zig" });
    defer testing.allocator.free(source_path);
    {
        const file = try std.fs.cwd().createFile(source_path, .{});
        defer file.close();
        try file.writeAll("pub fn sample() void {}\n");
    }

    const result = try std.process.Child.run(.{
        .allocator = testing.allocator,
        .argv = &.{ "zig", "build", "run", "--", tmp_root, "snapshot" },
        .cwd = "/Users/rachpradhan/codedb2",
        .max_output_bytes = 256 * 1024,
    });
    defer testing.allocator.free(result.stdout);
    defer testing.allocator.free(result.stderr);

    try testing.expect(result.term.Exited != 0);
}

Expected
codedb <tmp-root> snapshot and MCP codedb_index should reject temporary-directory roots before scanning or snapshotting them.

Fix
Add a shared root policy that rejects /private/tmp, /tmp, and similar temp roots in both the CLI snapshot path and the MCP indexing/cache path so these directories are neither indexed nor reloaded.

[justrach]

Fixed — root_policy.zig rejects temporary roots (PR #81). Verified: codedb tree on /tmp paths shows 'refusing to index temporary root'.