feat(cloud): active-target selection core (set_active_target)

[sourcehawk]

Description

Towards #44

Adds the provider-agnostic core that lets the operator agent pick which project (GCP) or account (AWS) subsequent run_cli commands run against, from a deployment-pinned set. The cloud MCP Server now holds the active target as in-memory session state and injects a provider-supplied env var into each run_cli child process, never via os.Setenv — the same per-exec discipline the identity probe follows. This is PR A of the feature: it lands the Provider interface extension, the server state and selectable-set logic, the set_active_target tool, and the run_cli guard, all driven against the in-package fake. The GCP/AWS account config and launcher wiring follow in PR B.

Changes

• Provider interface gains ConfiguredTargets() []Target and ActiveTargetEnv(targetID string) []string; new Target{ID,Name} type.
• Server tracks an activeTarget; selectableTargets resolves it from configured targets → scope.Projects → live inventory, and setActive rejects any id outside that set so the agent can never name an arbitrary target.
• subprocessEnv appends the provider's active-target env per exec; a single selectable target is default-active from session start.
• New set_active_target tool (registered between session_status and run_cli, added to ToolSpecs()); the wire test enforces the catalog match.
• run_cli returns an actionable error pointing at set_active_target when no target is active and more than one is selectable.
• session_status reports the active target via a new IdentityStatus.ActiveTarget field.
• The real gcp/aws providers implement the two new methods to keep the tree compiling (gcp: CLOUDSDK_CORE_PROJECT; aws: AWS_PROFILE); PR B fleshes out the AWS accounts/profile generation.

Challenges

Widening the Provider interface broke the compile-time conformance assertions in the gcp/aws providers and a probe test double. Rather than land a red build, this PR gives them the methods now — gcp in its final form, aws in the minimal correct shape that PR B's profile-name logic replaces.

Related

• PR B (provider impls + AWS accounts config + launcher wiring) builds on this.
• Design: docs/superpowers/specs/2026-05-31-cloud-active-target-selection-design.md.

Testing

make test-go (race-clean, 43 packages, 0 failures) and make lint (0 issues) both green. New tests in pkg/mcp/cloud cover: the fake satisfying the extended interface; selectableTargets across all three fallback sources; setActive membership rejection; subprocessEnv target injection; single- vs multi-target default-active; the set_active_target tool (valid + rejected ids); run_cli's no-active-target guard and its release once a target is set; and session_status reporting the active target. The wire test fails if set_active_target registration drifts from ToolSpecs(). No frontend/ changes.

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