docs: what an affordance is, and the test that catches the bad ones

the substance was in plugins/CLAUDE.md marked "DESIGN, NOT BUILT", which
stopped being true the same day it was written — load.ts reads them, publish.ts
copies a plugin's in on install, and claude-manager compiles them into every
session. moved to docs/affordances.md and corrected; plugins/CLAUDE.md keeps
the plugin-specific part and points here.

the new part is a third test, earned today by nearly failing it twice: CAN IT
BE FIXED INSTEAD? an affordance is for something irreducible about the
surface, and a defect is not irreducible.

both candidates drafted today failed it. "links open in the same tab" was one
missing line in MD_COMPONENTS. "AskUserQuestion never returns an answer" was
true when the code comment was written and already fixed by the time i read
it — that one would have shipped a permanent instruction to work around a bug
that no longer existed, in a file whose whole purpose is to be believed
without checking.

names the failure mode: an affordance describing a defect calcifies it. once
there is a document explaining the workaround, the defect has a rationale, and
the rationale outlives anyone's memory that it was a bug.

also records two things that are not built and read as if they were: nothing
is scoped, so every affordance is in every conversation, and affordanceBlock()
has exactly one caller — an opencode turn gets none at all.

CLAUDE.md

@@ -421,6 +421,9 @@ explaining why it was safe.
 
 - `docs/navigation-audit.md` — **authoritative** on routing/navigation: the opaque-click anti-pattern,
   a severity-ranked findings table, the channel-selection map and the four-phase plan
+- `docs/affordances.md` — what an affordance is, the three tests for whether something qualifies (including
+  the one that catches most bad ones: _can it be fixed instead?_), the file format, and how they reach the
+  agent. Read before writing one
 - `docs/agent-coordination.md` — **the north star** for the workspace/panel work: agents on one
   dashboard coordinating with each other instead of through the human, the handoff protocol, and what
   is deliberately _not_ being built. Read it before ranking, deferring or starting any panel item —