docs(plans): record the S15 step 15 MRs and its spec file

What this does

Two entries on S15's plan for Step 15 (observability hooks), both raised on !1878 (merged)'s review.

The Status row. Step 15's cell was empty while its three part MRs were open. Guardrail 4 asks the cell be recorded in the same sitting as the step MR opening, and that the table have a single writer rather than each step MR editing its own row. This is that writer: the row now names !1878 (merged), !1879 (merged) and !1880 (merged), in the order they merge.

The Files list. Step 15's label renames are what make the Observability table in docs/specs/S15-npm-remote.md go stale, and the spec was not in the step's Files. The plan's own No step owned the S15 spec amendments the re-cut dropped section is the rule that such an amendment gets a named owner instead of being dropped on the floor, so this closes the same gap that section records.

The entry is careful about what listing the file claims. It names the spec for traceability, not as a diff one of Step 15's own MRs carries: the amending edit rides !1882 (merged) because that MR already edits the same table, and two branches editing adjacent rows cannot both merge on a train. !1878 (merged) carried the rename until review moved it.

What this does not do

Nothing about Step 15's scope, acceptance criteria or tests changes, and no other step's row is touched. Step 16's cell stays empty, which means "not yet recorded".

Reviewing this

One file, 23 lines added. The Status row is the diff's last hunk.