fix(guardrails): pin library semantics of an empty-but-non-nil allowlist (disjoint-intersection edge)

[initializ-mk]

Follow-up from the #285 review (inline finding on guardrails_merge.go).

MergeGuardrails intersects the agent's and platform's urlFilter.allowlist (and skillConstraints.allowedSkills) — fewer entries = stricter. But when the two lists are disjoint, the intersection is an empty-but-non-nil slice. Whether the guardrails library reads that as:

• "deny everything" (the correct ratchet — nothing passes the allowlist), or
• "no allowlist filtering" (a catastrophic loosen — everything passes)

depends on how the library interprets an empty-but-non-nil allowlist under the active Mode. The current code special-cases "platform empty = no opinion" but NOT the disjoint case.

Ask

• Determine the library's actual behavior for Allowlist: []string{} (non-nil, empty) under allowlist / both modes — write a test pinning it.
• If empty means "allow all", the intersect MUST special-case disjoint → an explicit block-all (e.g. a sentinel deny), so a disjoint overlay never loosens.
• Same treatment for skillConstraints.allowedSkills.

A code comment marking the caveat is in place (guardrails_merge.go, urlFilter allowlist block); remove it once resolved.

Related: #284, PR #285.