refactor: eliminate duplicate code in DIFC labels and RPC logger

[Copilot]

Closes #2001, closes #2002, closes #2003

Summary

Addresses the duplicate-code analysis report by refactoring two patterns identified in issues #2002 and #2003.

internal/difc/labels.go (issue #2002)

Added a getLabel() *Label helper method to both SecrecyLabel and IntegrityLabel. The helper safely handles nil receivers and returns nil in that case. This eliminates four identical 6-line nil-guard extraction blocks that preceded every call to checkFlowHelper, and simplifies both Clone() methods.

• CanFlowTo and CheckFlow on both types now call l.getLabel() / target.getLabel() directly — the nil-unwrapping blocks are gone
• Clone() on both types uses l.getLabel() == nil to guard the early-return path (covers both nil-receiver and nil-Label cases)

internal/logger/rpc_logger.go (issue #2003)

Extracted an unexported newRPCMessageInfo constructor that builds an RPCMessageInfo and sets the Error field when the supplied error is non-nil. logRPCMessageToAll now calls this helper twice (once with MaxPayloadPreviewLengthText, once with MaxPayloadPreviewLengthMarkdown) instead of repeating the struct literal + if err != nil block.

Impact

• ~21 net lines removed (95 → 74 in labels.go region; 38 → 22 in rpc_logger.go region)
• No behaviour change; all existing tests pass (make agent-finished ✓)
• Future additions to RPCMessageInfo fields or nil-guard logic now have a single place to update

[Copilot]

Pull request overview

Refactors two previously duplicated patterns to reduce boilerplate and centralize logic in DIFC label flow checks and RPC message logging, aligning with the duplicate-code analysis issues (#2002, #2003).

Changes:

• Added getLabel() *Label helpers on SecrecyLabel and IntegrityLabel to remove repeated nil-unwrapping blocks and simplify Clone().
• Extracted newRPCMessageInfo helper to avoid duplicated RPCMessageInfo construction in logRPCMessageToAll.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/difc/labels.go	Adds getLabel() helpers and refactors flow-check and clone methods to use them.
internal/logger/rpc_logger.go	Adds newRPCMessageInfo constructor and uses it to build text/markdown log payloads without duplication.

Comments suppressed due to low confidence (1)

internal/difc/labels.go:310

• Same as the SecrecyLabel version: this comment implies nil is only possible when the receiver is nil, but l.Label may also be nil. Please adjust the comment to mention both cases to keep docs accurate.

// getLabel returns the underlying Label, or nil if the receiver is nil.
func (l *IntegrityLabel) getLabel() *Label {
	if l == nil {
		return nil
	}
	return l.Label

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