Rust refactor: consolidate codex+opencode harness adapters; drop WatcherController newtype

[willwashburn]

Context

Two separate cleanup wins sit in crates/relayburn-cli/src/harnesses/:

1. harnesses/codex.rs and harnesses/opencode.rs are 95% identical.

Both expose config() + adapter() with the same shape; they differ only in:

• harness name string,
• session_root path (a static ~/.codex/sessions vs ~/.local/share/opencode/storage/session),
• ingest function (ingest_codex_sessions_into vs ingest_opencode_sessions_into).

These adapters are already constructed via harnesses/pending_stamp.rs::createPendingStampAdapter per the project convention; the per-harness file is essentially a parameter pack.

2. harnesses/mod.rs:179-208 WatcherController is pure boilerplate.

It's a newtype around sdk::WatchController with three pass-through methods plus an unused raw(). The doc comment admits "today this is just a newtype" and enumerates speculative future use cases that haven't landed. Use sdk::WatchController directly — saves ~30 LOC and a wrapper allocation per start_watcher call.

Proposed fix

1. Add pub fn adapter_static(name: &'static str, session_root: &'static Path, ingest: fn(...) -> ...) -> impl HarnessAdapter to harnesses/pending_stamp.rs (or extend the existing factory).
2. Replace harnesses/codex.rs and harnesses/opencode.rs with one call each, or collapse to a single pending_stamp_harnesses! macro / one struct literal each. Both files become roughly 5 lines.
3. Replace WatcherController with sdk::WatchController at the call sites (harnesses/mod.rs:65, 75, ...). Drop harnesses/mod.rs:179-208.

Adjacent: harnesses/claude.rs:50, codex.rs:36, opencode.rs:39 triple-define the same let home = std::env::var_os("HOME").map(PathBuf::from).unwrap_or_else(|| PathBuf::from(".")). Hoist into util/.

Verification

cargo test -p relayburn-cli plus the harness-substrate unit tests under harnesses/. The runtime_adapter_names_match_runtime_adapters test is the existing parity check.

References

• crates/relayburn-cli/src/harnesses/{codex,opencode,mod,pending_stamp}.rs
• CLI review notes from the May 2026 Rust review.