hive-forge: exit after HIVE_FORGE_DEADLINE instead of hanging on a stalled forge

forgejo-api 0.11.1's sync client builds its own reqwest client with no
timeout and takes no caller-supplied client (sync.rs: new and
with_user_agent are the only constructors), so a forge that accepts a
connection and then stalls blocked every typed verb forever.

A watchdog thread now starts before the client is built. After
HIVE_FORGE_DEADLINE seconds (default 900, five minutes above
DOWNLOAD_TIMEOUT) it prints a hive-forge: FAILED: line and exits 75.
It bounds the whole command, not each request, and a write may already
have landed when it fires; the help text and docs/tools/forge.md say so.

docs/tools/forge.md

@@ -20,6 +20,12 @@ markdown-docs > docs/tools/forge-cli.md`.
   > the `origin` remote of the cwd's git checkout > `$HIVE_FORGE_REPO`
   (last-resort override, unset by default) > a hard error.
   `client::Client::from_env` assumes no single repo by default.
+- Deadline: one invocation runs for at most `$HIVE_FORGE_DEADLINE` seconds
+  (default 900, five minutes above the largest per-request budget). Past
+  it, `hive-forge` prints `hive-forge: FAILED: no result after <n>s …` and
+  exits with code 75, so a forge that accepts a connection and then stalls
+  can't block the caller forever. The command stops wherever it was: a write verb
+  may already have reached the forge, so check its result before retrying.
 
 ## Verbs