docs(adr): correct lineage validation reference

[Marco (marco13-moo)]

Description

Refreshes the ADR lineage reference and aligns it with the behavior currently implemented by validate_frontmatter.py and update_lineage.py.

The previous documentation described several safeguards as automated validation even though they are presently workflow policies. It also documented stable LINEAGE_* error categories that the scripts do not emit.

Changes

• Clarify which lineage requirements are enforced programmatically and which remain workflow responsibilities.
• Document that a superseder's status remains unchanged while the predecessor becomes superseded.
• Correct the last_decision_id guidance: the allocator validates and increments the configured value but does not reconcile it against ADR files on disk.
• Replace the nonexistent LINEAGE_* error-category list with the scripts' actual human-readable failure modes.
• Refresh the document review date after verifying the reference against the implementation and tests.

Impact

Contributors using the ADR author skill receive an accurate account of its guarantees and do not rely on validation that is not currently implemented.

Validation

• Reviewed the reference against scripts/update_lineage.py, scripts/validate_frontmatter.py, and their targeted test modules.
• git diff --check

Closes #2584

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