feat(nodes): add Slack Events source node

[kgarg2468]

Problem Statement

RocketRide pipelines cannot currently receive authenticated Slack Events API callbacks as a native ingress source. Users need a supported way to route Slack messages, mentions, and reactions into canvas pipelines without building a custom webhook adapter.

Proposed Solution

Add a slack source node that:

• Exposes /slack/events while the pipeline is running.
• Verifies Slack HMAC-SHA256 signatures and request timestamp freshness before parsing JSON.
• Handles authenticated URL verification challenges.
• Routes app mentions and supported message events to the text lane.
• Routes reaction_added events to the JSON lane.
• Preserves the complete outer Slack envelope as native entry metadata.
• Uses bounded in-memory queueing, event-ID deduplication, retry-friendly HTTP responses, and graceful shutdown.
• Includes tests, documentation, a native Slack icon, and an example pipeline.

Alternatives Considered

• A generic webhook node: requires every user to reproduce Slack signature verification, event classification, retry behavior, and feedback-loop prevention.
• Socket Mode: adds a persistent outbound connection and app-token lifecycle that is outside the intended HTTP Events API scope.
• Extending tool_slack: that node is an outbound agent tool, while this feature is an inbound pipeline source.

Affected Modules

• nodes (pipeline)

[github-actions]

🤖 Internal: Discord sync marker

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