chore: add changesets for the five changed protocol packages

Minor bumps for @atrib/mcp, @atrib/verify, @atrib/emit, @atrib/agent,
and @atrib/mcp-wrap covering the D137-D142 additive opt-in surfaces.

Author: creatornader

.changeset/tranche2-agent-extension-attribution.md

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+---
+'@atrib/agent': minor
+---
+
+Add client-side `dev.atrib/attribution` extension support per [D141](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d141-devatribattribution-first-class-mcp-extension-sep-2133): declare the extension on outbound requests and parse attestation receipts from `result._meta`, behind an opt-in flag with unchanged behavior when unset.

.changeset/tranche2-emit-session-checkpoint.md

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+---
+'@atrib/emit': minor
+---
+
+Add `emitSessionCheckpoint` per [D139](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d139-session_checkpoint-event-type-the-session-stream-formalized): sign a session-checkpoint record committing to the context's RFC 6962 session root through the existing emit pipeline, silent-failure per the [§5.8](https://github.com/creatornader/atrib/blob/main/atrib-spec.md#58-degradation-contract) degradation contract.

.changeset/tranche2-mcp-anchors-checkpoints-delegation-extension.md

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+---
+'@atrib/mcp': minor
+---
+
+Add four opt-in protocol surfaces: the anchors module per [D138](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d138-anchor-plurality-as-the-default-trust-posture) (anchor-set posture resolution, anchoring-claim builder with fresh Ed25519 signatures, non-blocking multi-anchor fan-out with the atrib-log transport live and rekor/rfc3161/ots transports stubbed behind `AnchorTransport`), session checkpoints per [D139](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d139-session_checkpoint-event-type-the-session-stream-formalized) (RFC 6962 session roots, checkpoint record assembly under the extension URI, inclusion and consistency proofs, equivocation detection), delegation-certificate issuance per [D140](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d140-delegation-certificates-principal-keys-certify-ephemeral-run-keys) (`issueDelegationCertificate`, `withDelegationCertHash`, run-key revocation builder), and the `dev.atrib/attribution` extension surface per [D141](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d141-devatribattribution-first-class-mcp-extension-sep-2133) (capability negotiation including legacy-initialize gating, attestation receipt building and application, middleware `extensionAttribution` flag). Middleware and proxy behavior is byte-identical when none of these are configured.

.changeset/tranche2-mcp-wrap-extension-flag.md

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+---
+'@atrib/mcp-wrap': minor
+---
+
+Add the `extensionAttribution` config flag per [D141](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d141-devatribattribution-first-class-mcp-extension-sep-2133), letting the wrapper declare the `dev.atrib/attribution` extension on behalf of any upstream server; wrapping behavior is unchanged when the flag is unset.

.changeset/tranche2-verify-envelope-anchors-checkpoint-delegation.md

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+---
+'@atrib/verify': minor
+---
+
+Add the universal evidence-envelope module per [D137](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d137-universal-evidence-envelope-as-the-single-protocol-level-attachment-model) (`validateEnvelope`, the atrib-maintained profile registry, the frozen legacy-protocol mapping, opaque rendering) with an opt-in `verifyRecord({ evidenceEnvelopes })` projection; the `anchor_plurality` verifier annotation per [D138](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d138-anchor-plurality-as-the-default-trust-posture); session-checkpoint verification per [D139](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d139-session_checkpoint-event-type-the-session-stream-formalized) (structural validation, freshness tiering, full verification); `evaluateDelegation` with an opt-in verifyRecord delegation block per [D140](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d140-delegation-certificates-principal-keys-certify-ephemeral-run-keys); and the `continuation-packet` profile as the ninth registry entry per [D142](https://github.com/creatornader/atrib/blob/main/DECISIONS.md#d142-orchestration-topology-baton-pass-and-join-records-as-attest-conventions). Evidence remains signal-not-block: `verifyRecord().valid` semantics are unchanged.