fix: support OpenCode v2 plugin lifecycle

[Grivn]

OpenCode 2 rejects the installed Mnemon plugin's V1-only named export, so conversations continue without recall, shell environment setup, or compaction guidance. Export one { id, setup, server } definition: setup uses the V2 hook contracts, while server keeps compatibility with OpenCode 1.18.29 and newer.

Closes #150.

Recall is attached to the latest user message on every outgoing model request. Fresh tool-continuation copies reuse the current turn's recall; session isolation, a bounded cache, and unload cleanup prevent stale or repeated injection. CLI execution uses argument arrays and a hard timeout, resolving standard npm installs to their native executable so a timed-out launcher cannot leave its child running. The setup notes explain the supported versions and how to refresh existing installations.

Verified against real OpenCode 2.0.18 processes with the original and fixed templates installed by mnemon setup, an isolated synthetic store, and a local deterministic provider:

Observation	Original template	Fixed template
Plugin state	Loader error, no active Mnemon plugin	Exactly one active Mnemon plugin
Two sessions, three turns, six model requests including real shell continuations	No memory context	Exactly one context block with the recalled fact in each request
Mnemon calls	None	One status/recall pair per turn
Actual shell command	Missing environment marker	MNEMON_OPENCODE=1
Completed compaction	No Mnemon guidance	Guidance included in the provider request
Persisted conversation	No injected context	Still no injected context

Validation:

• go build -o mnemon . and make test pass.
• go test ./test/memory/opencode -v -count=1 passes, including real SIGTERM-ignoring child processes for direct and npm installation paths.
• Real OpenCode 1.18.29 also loads exactly once and includes recall in both initial and tool-continuation requests.
• Independent CLI smoke, local/global setup/reinstall/eject, and Memory CLI E2E pass (258/258 checks).

The callback and process regressions remain in the explicit integration tier. Actual host runs used macOS arm64; Windows command resolution is covered by mocked path fixtures. All model traffic uses the local fixture; no paid provider is required. The older V1 loader support floor is documented as 1.18.29.