httpdl/cli: drop --auto-split; classify errors by type, not message text

Two Pike cleanups.

--auto-split was a got-only third connection-count policy (beside -x and -s)
that invented rather than matched aria2. Removing it also retires the now-dead
autoConns/maxAutoConns and the per-host transport cap that existed only to scale
for it; min(split, M*-x) is the sole policy again.

main's exit-code mapping fell back to substring-matching third-party error text
(strings.Contains "connection refused"/"timeout"/...), which rots when a
dependency rewords a message. The typed checks (errors.Is on the syscall errno,
*net.DNSError, net.Error.Timeout, context.DeadlineExceeded) already cover the
real stdlib errors -- verified end to end: refused -> 6, bad host -> 19. The two
cases that genuinely needed the text match are our own errors, now typed
sentinels: httpdl.ErrTimeout (idle timeout) and the bt metadata timeout wrapping
context.DeadlineExceeded.

README.md

@@ -18,9 +18,6 @@ Needs Go 1.25+.
 # HTTP download with 16 connections
 got -x16 -s16 https://example.com/big.iso
 
-# let got pick the connection count from the file size
-got --auto-split https://example.com/big.iso
-
 # resume an interrupted download
 got -c https://example.com/big.iso
 
@@ -47,7 +44,6 @@ Run `got --help` (or `--help=all`) for every option.
 | `-c, --continue` | resume a partial download | false |
 | `-x, --max-connection-per-server` | connections to one server (1–16) | 1 |
 | `-s, --split` | split a download into N connections | 5 |
-| `--auto-split` | choose connections from file size (overrides `-x`/`-s`) | false |
 | `-j, --max-concurrent-downloads` | downloads at once | 5 |
 | `--max-overall-download-limit` | global speed cap | 0 (off) |
 | `--checksum` | verify the finished file: `TYPE=DIGEST` (sha-256, sha-1, …) | |