docs(caveats): describe shipped caveat wiring, not "next step"

[hartsock]

What this PR does

• Fixes stale prose in the agent-mesh-protocol/src/caveats.rs module doc that claimed wiring caveats into AgentMetadata and enforcing child ⊑ parent "is the next step." Both have shipped and are property-tested; the old text made an enforced security property read as aspirational.
• Rewrites the paragraph to describe the shipped state: the caveats field is part of the signed cert payload, and attenuation is enforced at two independent points — AgentKey::delegate refuses to mint an amplifying child (MeshError::CaveatAmplification), and CertChain::verify re-checks attenuation at every delegated link, so a forged chain is rejected even when each signature is valid.
• Preserves the genuine forward reference: OS-level enforcement (Landlock, uid-mapped namespaces) is the remaining future work, and the pointer to docs/decisions/agentic_object_capability_security.md in newt-agent is kept.
• Adds intra-doc links to AgentKey::delegate, CertChain::verify, and MeshError::CaveatAmplification (all crate::-qualified, resolve cleanly).

Test plan

• Documentation-only change — no code or test changes.
• just check passes locally: cargo fmt --all -- --check clean, cargo clippy --workspace --all-targets -- -D warnings clean (zero warnings), and the full cargo test --workspace suite is green.
• The enforcement tests the new doc references remain green and unchanged: forged_amplifying_chain_fails_verify, delegate_rejects_amplification, multi_level_delegation_attenuates_each_link, tampered_caveats_fails_verify (all in agent-mesh-protocol/src/agent_key.rs).
• Verified my new intra-doc links produce no rustdoc warnings on caveats.rs.

Out of scope

• OS-level enforcement (Landlock, uid-mapped namespaces) remains genuinely future work and is described as such in the doc.
• No changes to the confidentiality model or any other module docs; only the caveats.rs module doc was out of date.

Fixes #41