refactor: replace single-variable mutexes with atomic types

[dgageot]

Each of the three structs changed here used a sync.Mutex or sync.RWMutex to protect exactly one mutable field — all other fields were written once at construction and never changed afterward. When a mutex guards a single variable, the standard library's sync/atomic package offers a simpler and more self-documenting fit: the data and its synchronization are expressed as one value rather than two separate declarations.

lspSession in pkg/tools/builtin/lsp/lsp_lifecycle.go had a closed bool protected by a plain mutex. It is now an atomic.Bool, with CompareAndSwap in Close() to preserve idempotency and Load() in the wait goroutine to check the flag. Client in pkg/model/provider/rulebased/client.go had a lastSelectedID of type modelsdev.ID behind an RWMutex; it is now an atomic.Pointer[modelsdev.ID]. agentRouter in pkg/runtime/agent_router.go had a current string behind an RWMutex; it is now an atomic.Pointer[string].

All changes are behavior-preserving. The suite passes under go test -race and the linter reports no new issues.

[docker-agent]

Assessment: 🟢 APPROVE

The refactor correctly replaces single-variable mutexes with atomic types across all three files:

• **** → with in for idempotency and in the wait goroutine — both uses are correct.
• **** → — nil checks are in place before dereferencing the loaded pointer.
• **** → in — nil is handled correctly.

The removed mutexes each guarded exactly one mutable field (as described), so no other fields are left unprotected. The changes are behavior-preserving and idiomatic Go.

[docker-agent]

Assessment: 🟢 APPROVE

The refactor correctly replaces single-variable mutexes with atomic types across all three files. No bugs were found in the changed code.

• lspSession.closed → atomic.Bool: CompareAndSwap(false, true) in Close() correctly preserves idempotency; Load() in the wait goroutine is the right read primitive.
• lastSelectedID → atomic.Pointer[modelsdev.ID]: nil is handled before dereferencing the loaded pointer.
• current → atomic.Pointer[string] in agentRouter: nil is handled correctly on load.

The removed mutexes each guarded exactly one mutable field, so no other fields are left without synchronization. The changes are behavior-preserving and idiomatic Go.

[docker-agent]

Assessment: 🟢 APPROVE

The refactor correctly replaces mutex-protected single fields with sync/atomic types across all three files.

lspSession.closed → atomic.Bool: The CompareAndSwap(false, true) in Close() correctly preserves idempotency. The happens-before ordering (CAS → processCancel → process exit → cmd.Wait()) ensures the wait goroutine observes closed=true after a signal-induced exit.

Client.lastSelectedID → atomic.Pointer[modelsdev.ID]: Taking the address of the local selectedID variable correctly heap-escapes it via the Go compiler. The nil-check on Load() properly mirrors the old zero-value behavior on first call.

agentRouter.current → atomic.Pointer[string]: newAgentRouter always calls Store before the struct is shared, and the nil guard in Name() is a correct and harmless safety net.

No bugs introduced. All changes are behavior-preserving.