feat: add discover-local-peers multi-log heartbeat aggregation CLI

[Exohayvan]

Objective

Add a read-only CLI command that builds a single local peer roster from multiple heartbeat announcement logs.

This is important because it lets operators combine discovered local nodes from separate announcement files into one deterministic view before introducing any real networking, while keeping discovery file-backed and local-only.

What changed

• Added discover_local_peers_document in peer_registry to aggregate heartbeat peers across one or more local message-log inputs using existing message validation/parsing.
• Refactored peer_summary_document to delegate to the shared multi-log helper so single-log and multi-log behavior stays aligned.
• Added deterministic merge behavior for duplicate node IDs: highest heartbeat_sequence controls visible state, equal sequences keep first-seen visible values, and heartbeat_count counts all valid heartbeats across inputs.
• Added CLI discover-local-peers subcommand with repeatable --message-log-path arguments, read-only output-only behavior, and existing PeerRegistryError handling.
• Extended parser help and command registration snapshots to include the new command.
• Added peer-registry and CLI tests for multi-log aggregation, duplicate merges, equal-sequence ties, malformed input errors, non-creation/immutability on failure, and expected usage/help behavior.
• Updated README with an example showing two announce-local-node calls followed by discover-local-peers.

Validation

• Executed: PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=src python -m unittest discover -s tests -v
• Result: all tests passed (306 tests, OK) including the new peer discovery unit and CLI coverage.
• Validation evidence is from the locally run test command/output for this branch.

Safety notes

• No force push used.
• No hard reset used.
• Local automation paths are not published in this PR body.

[Exohayvan]

Closing during PR cleanup: this peer discovery/roster helper now conflicts with main after #50 merged . The merged command covers the active peer-log dispatch path, so this older overlapping PR is not being kept alive.