XY-519 Add public Linear text guard

[yvette-carlisle]

Summary

• Add a shared public text guard for Linear comment bodies and execution event free-text fields.
• Route internal retry, manual closeout, and Linear execution event writes through the guard before tracker writeback.
• Add tracker public-boundary prompt guidance and update the governing specs.

Verification

• cargo test -p decodex
• cargo make checks

Note

• Decodex automation for the private-ledger rollout remains paused until this guard is landed and the running runtime is updated.