docs(skill): scope inactive visibility to what a source can attest (#36)

`inactive` is populated only by Pi. The skill described it as a Pi
capability but never said what its absence means elsewhere, so an agent
querying a Claude Code session with `includeInactive: true` reads an empty
result as "nothing was abandoned here". Across 43 local Claude transcripts,
20 contained branches off the surviving path, so that reading is wrong far
more often than it is right.

Claude Code and Codex transcripts record no supersession event to attest
from; Kimi records one but the adapter drops those messages while indexing.
The added sentence covers all three without naming them, so it stays correct
if Kimi later marks retracted history instead of discarding it.

skill-doc/SKILL.md

@@ -173,7 +173,10 @@ messages unless `includeMeta: true` is passed; `context()` and `trace()` preserv
 the current causal chain and expose `is_meta` on returned rows.
 
 Pi can preserve a branch that was tried and later superseded as
-`visibility='inactive'`. Default helpers return only `visible` evidence. Pass
+`visibility='inactive'`. Only Pi populates it: other sources either do not
+record supersession in their transcripts or discard it while indexing, so an
+empty inactive result never means nothing was abandoned -- only that this
+source cannot say. Default helpers return only `visible` evidence. Pass
 `includeInactive: true` to `search()`, `context()`, `trace()`, `thread()`,
 `summaries()`, `raw()`, `fileHistory()`, or `failures()` only when the abandoned
 path matters. Every returned message or evidence row is labeled with