test(posture): restore stance-claim enforcement lost when internal/gascity removed (ag-ddrb #posture-tests)

[boshu2]

What

Adds cli/internal/posture/posture_test.go — the package had zero tests.

Why

internal/posture/posture.go is pure and deterministic but its doc comment promises:

the AgentOps stance claim "must either update this value or fail the integration test in cli/internal/gascity/bridge_test.go"

internal/gascity was deleted (ag-hfc, 3.1 teardown — confirmed in CLAUDE.md), so that enforcing test no longer exists. The AgentOps stance declaration has been unguarded since. TestAgentOpsStance here is the replacement regression guard: a silent edit to the stance now fails CI.

Coverage

Behavioral, table-driven, 100.0% statement coverage (10 tests):

• Layer.String() — all six + sentinel/out-of-range
• Layer bit-position values (the shift-amount contract LayerSet depends on)
• LayerSet NewLayerSet/Has/Union — idempotence, commutativity, empty-set identity
• AllLayers / ExternalLayers exact membership ("Bead is universal")
• Kind.String() — all families + out-of-range
• AgentOps stance invariant: Kind=Distribution, owns exactly {Role,Skill,Ledger,Distribution}, does not own Loop, Bead is external
• owned ⊆ AllLayers structural invariant

No coverage-padding: every assertion checks exact behavior or a documented invariant.

Verification

go test ./internal/posture/ -cover  → ok  100.0% of statements (10 pass)
go vet ./internal/posture/          → clean
golangci-lint run ./internal/posture/ → 0 issues
gofmt -l                            → clean

Closes-scenario: ag-ddrb#posture-tests
Bounded-context: BC5-Runtime
Evidence: go test ./internal/posture/ -cover → 100.0%