fix(route): measure the effort a session ran with, instead of asking for it

The last-session record exists to make the routing policy falsifiable, and
it only is if --last-effort is measured. route.sh documented the opposite as
settled fact: that the effort a session ran with is not observable from
inside that session. That was true when written and is not now. Claude Code
exports CLAUDE_EFFORT into every tool-use context as the session's current
effort level, so a Bash call reads it directly.

The premise had a cost. With effort unobservable, the record could only be
completed by asking the operator at session end, which made it block on their
presence -- all four fields or none. That is also the weaker measurement, and
in the same way the previous board line is: the operator reads the effort off
the startup command they typed, so both sources report what was PRESCRIBED
rather than what was RUN. They come apart exactly when the record would be
most interesting, which is what a session that silently ran xhigh under a
board line saying high already showed.

Reading it makes all four fields knowable from inside the ending session, so
the record no longer waits on anyone. The skill does the reading; route.sh
deliberately does NOT default from the variable, because a calculator that
consults its environment is no longer deterministic from its arguments and
the route->board round trip in selftest section 6 rests on that.

Section 13 also pins the trap this opens: skill frontmatter overrides the
session effort while that skill is active, so an effort: field in route's own
SKILL.md would make the reading report the skill instead of the session --
a measurement quietly measuring itself, with nothing in the output to show
it happened.

Also corrects the neighbouring claim that the model is readable from the
environment. There is no CLAUDE_MODEL; the session takes it from what it
knows itself to be running as.

route-selftest 50 -> 56.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gfa1nvwGXdST2MHvbs6htD

scripts/route.sh

@@ -91,11 +91,16 @@
 # zero on row 4 means escalation fires too readily.
 #
 # All four fields are closed sets or numbers, and required together, because the
-# NEXT session reads the record back to decide --opus-xhigh-failed. Note what
-# the caller must supply rather than infer: the effort a session ran with is not
-# observable from inside that session, and reading it back off the previous
-# board line would measure what was PRESCRIBED, not what was RUN. Omit the
-# record rather than guess - a guessed value reads back as a measurement.
+# NEXT session reads the record back to decide --opus-xhigh-failed. Every one of
+# them must be MEASURED by the caller: --last-effort comes from CLAUDE_EFFORT,
+# which Claude Code exports into every tool-use context as the session's current
+# effort level. It is deliberately NOT defaulted from that variable here - a
+# calculator that reads its own environment stops being deterministic from its
+# arguments, and section 6 of the selftest depends on that determinism. Reading
+# the value off the previous board line would measure what was PRESCRIBED rather
+# than what was RUN; so, less obviously, does asking the operator, who reads it
+# off the startup command they typed. Omit the record rather than guess - a
+# guessed value reads back as a measurement.
 #
 # It is deliberately NOT wired to the Fable rows. Inferring "the model failed"
 # from "the session did not finish" would fire on context exhaustion and on