docs: report MR target discovery blocker

docs/report/merge-request-open-ticket-target-discovery.md

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+# Merge Request open requires an undiscoverable Ticket repository target
+
+## Observed
+
+While implementing Ticket `00001KZRNHB35`, the assigned Coder had a clean committed Workdir and all immutable base/head/changed-path evidence needed by `MergeRequestOpen`.
+
+`MergeRequestOpen` rejected candidate repository ids with:
+
+```text
+invalid input: Merge Request repository must match the authoritative Ticket target
+```
+
+The typed `TicketShow` result available to the Coder rendered only the Ticket id and state; it did not expose the authoritative `repository_id` or ref selector. No typed repository/workdir lookup tool was available to the Coder. Continuing would therefore require guessing control-plane identity, mutating the Ticket target without evidence, or bypassing typed authority, all of which are correctly prohibited.
+
+## Impact
+
+A Coder can finish and validate implementation but cannot open the required immutable MR revision or start independent review. The failure is safe, but it strands otherwise review-ready work and provides no actionable expected target.
+
+## Suggested improvement
+
+At least one trusted read surface in the assigned-Coder flow should return the immutable Ticket target needed by `MergeRequestOpen`:
+
+- include `repository_id` and `ref_selector` in `TicketShow`'s bounded authoritative projection; or
+- have `MergeRequestOpen` derive repository identity from the authoritative Ticket target and remove it from model input; or
+- return a bounded structured mismatch diagnostic containing the authoritative repository id when the caller is already authorized to read that Ticket.
+
+Deriving the repository in `MergeRequestOpen` is preferable because it removes duplicated model-controlled identity and avoids target drift between Ticket read and MR creation.