feat(runtime): implement signed checkpoint lineage (Task 13)

[simonovic86]

Summary

Implements cryptographic signed checkpoint lineage enabling verifiable checkpoint authenticity and divergent lineage detection without trusting the executing node.

Key Changes

New Packages

• pkg/identity/ — Agent Ed25519 keypair generation, serialization, and persistent storage
• pkg/lineage/ — Signed checkpoint types, content hashing (SHA-256), Ed25519 signing/verification, and signing domain construction

Checkpoint Format v0x04

• Extended v0x03 header (81 bytes) with 128 bytes of lineage metadata:
  • PrevHash [32]byte — SHA-256 of previous checkpoint (hash chain)
  • AgentPubKey [32]byte — Agent's Ed25519 public key
  • Signature [64]byte — Ed25519 signature over header + state
• Total header: 209 bytes
• Backward compatible: reads v0x02/v0x03, writes v0x04 when identity set, v0x03 when disabled

Signature Protocol

• Signing domain covers everything except signature slot: checkpoint[0:145] || state
• Sign flow (SaveCheckpointToStorage): build checkpoint, construct signing domain, sign with private key, embed signature, compute content hash for next checkpoint
• Verify flow (LoadCheckpointFromStorage): extract public key and signature from checkpoint, reconstruct signing domain, verify with Ed25519

Agent Identity Management

• Identity generated on first load, persisted as <agentID>.identity
• Transferred during migration via AgentPackage.AgentIdentity
• Deleted from source after confirmed migration

Storage Provider Extension

• SaveIdentity(ctx, agentID, data) — Persist agent identity
• LoadIdentity(ctx, agentID) — Load persisted identity

Instance Changes

• Added AgentIdentity and PrevCheckpointHash fields
• buildCheckpoint() handles v3 (no lineage) and v4 (signed) formats
• ParseCheckpointHeader() refactored to return CheckpointHeader struct

Invariant Support

• EI-3 (Checkpoint Lineage Integrity): PrevHash chain + signatures
• EI-11 (Divergent Lineage Detection): conflicting PrevHash values prove forks
• OA-1 (Canonical Logical Identity): agent public key in each checkpoint
• MC-3 (Migration Continuity): identity and hash chain preserved across nodes

Documentation

• Added docs/runtime/SIGNED_LINEAGE.md with protocol, format, signing flow, and security considerations
• Updated IMPLEMENTATION_STATUS.md to mark Task 13 complete
• Updated CLAUDE.md with checkpoint v0x04 format details

Testing & CLI

• Refactored cmd/igord to load/generate identity via loadOrGenerateIdentity()
• Added initLocalAgent() helper for local agent setup
• Inspector updated to parse and verify v0x04 signatures
• All checkpoint parsing tests updated to use new CheckpointHeader struct

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