harness-tools: read, write, bash, glob, grep

Five of the six M1 built-ins (edit's replacer chain is a separate port).
bash uses process_group(0) + SIGKILL-the-group on timeout/cancel; glob/grep
are gitignore-aware via ignore::WalkBuilder and don't ask permission
(read-only); read/write/bash ask through ctx.ask with opencode's coarser
"always" pattern (first word + wildcard for bash, path for edit/read).
Wired tool output through the central 30k-char truncate() in the processor
so every tool gets spill-to-disk behavior for free. 70 tests passing,
clippy clean.

Cargo.toml

@@ -57,6 +57,7 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tracing-appender = "0.2"
 dirs = "5"
 shell-words = "1"
+libc = "0.2"
 tempfile = "3"
 insta = "1"
 wiremock = "0.6"