fix(ci): unwire test-start-agent-session.sh, restore its signed exclusion (#1269)

The `test` step has failed on every `next` pipeline since #1017 on exactly one
assertion, and it is the same one on unrelated PRs:

    FAIL: host provides 'pi' in the system path; missing-binary cases are not
    measurable here            (framework/tools/fleet/test-start-agent-session.sh:103)

Measured 2026-08-16 across pipelines 2444 (#1256), 2438 (#1240) and 2441
(#1017-quality): exactly one FAIL line in each full log, identical, this line.
Control `zzz-not-present-zzz` -> 0 on all three.

Cause. #1241 (5c35a250) added the guard: the suite shims fake mosaic/pi/npm into
$FAKE_BIN, but the constructed PANE_PATH always ends in the real system path, so
on a host that installs those binaries the missing-binary cases cannot be
measured and a green run would mean nothing. The guard says so instead of
passing. Its own pipeline 2430 was green only because the suite was CI-excluded
at the time, so the guard had never run in CI. #1017 (c56483eb) then enumerated
it and dropped the exclusion. The CI image installs
@earendil-works/[email protected].1 on purpose, so the precondition is
unsatisfiable there. Both commits are mine.

The guard is correct and is not being softened. A check that cannot measure its
property and reports success is the failure mode this repo has been cataloguing
all week; the error was wiring the suite into an image that violates its
precondition, so the wiring is what gets reverted.

Second effect, which is the reason this cost a day rather than an hour:
test:framework-shell is one && chain and this sat at position 44 of 48, so
glpi/test-list-http-status.sh, orchestrator/test-board-roll.sh,
woodpecker/test-ci-wait-exit-matrix.sh and _scripts/test-fleet-transport-check.sh
have not run at all since the merge. The pipeline reported one failure, never
"one failure plus four unrun". All four are green when run directly on
sb-it-1-dt, so the mask hid nothing broken -- but that is a local result on one
host, not a CI-image result.

Verification, with controls:
- enumeration guard OK (population 52, enumerated 36, signed-excluded 16).
- control A, exclusion line removed while unwired -> FAIL UNENUMERATED.
- control B, exclusion line kept while rewired -> FAIL CONTRADICTORY EXCLUSION.
  The gate discriminates in both directions, so its OK is load-bearing.
- the four formerly-masked suites: rc=0 each, run directly.
- the full chain cannot be run to completion on sb-it-1-dt: it stops earlier, at
  the lease-broker Invariant R test, because this host carries the quarantined
  operator-global pi 0.84.2 against a measured 0.84.1. That is host-specific and
  out of scope here -- CI pins 0.84.1, and the single FAIL line in those three
  pipelines proves positions 1-43 passed there.

Burn-down is to control the tail of PANE_PATH inside the test, not to remove pi
from the image. Recorded in the exclusion reason and in #1269.

packages/mosaic/framework/tools/quality/test-enumeration-exclusions.txt

@@ -39,3 +39,19 @@ packages/mosaic/framework/tools/tmux/test-send-message-verdict.sh | requires rea
 # recorded judgement. These lines ARE that judgement, signed.)
 packages/mosaic/framework/tools/orchestrator/smoke-test.sh | behavior smoke checks for coord continue/run workflows, run manually by orchestrator seats; unmeasured in CI; #1017 burndown
 packages/mosaic/framework/tools/wake/validate-973/microtest-wake-assert.sh | #973 instrument self-test, run as a precondition of the validate-973 evidence procedure rather than as a standing CI suite; #1017 burndown candidate
+
+# --- tools/fleet: precondition is unsatisfiable in the CI image (#1269) ---
+# Signed by fred (sb-it-1-dt, 2026-08-16) at origin/next 476db12.
+# This suite asserts the launcher's behaviour when `mosaic` and `pi` are MISSING.
+# It shims fakes into $FAKE_BIN, but the constructed PANE_PATH always ends in the
+# real system path, so on a host that installs those binaries the missing-binary
+# cases cannot be measured at all. The suite's own guard (line 103) says so and
+# fails rather than reporting a pass it cannot back. That guard is correct.
+# The error was wiring the suite into CI: #1017 (c56483eb) enumerated it and
+# dropped this exclusion, and the CI image provides `pi` in the system path, so
+# it has failed on every pipeline since. Measured 2026-08-16 across pipelines
+# 2444 (#1256), 2438 (#1240) and 2441 (#1017-quality): exactly one FAIL line in
+# each full log, identical, this assertion; control `zzz-not-present-zzz` -> 0.
+# Because test:framework-shell is one && chain and this sat at position 44 of 48,
+# the four suites after it had not run at all since the merge.
+packages/mosaic/framework/tools/fleet/test-start-agent-session.sh | precondition unsatisfiable in the CI image: asserts missing-binary behaviour, but PANE_PATH always ends in the system path and the image provides `pi` there; guard at line 103 fails by design rather than passing unmeasured. Burn down by controlling the tail of PANE_PATH inside the test. NOT by removing `pi` from the image: the CI image installs @earendil-works/[email protected] deliberately (measured in pipeline 2444's test-step log), and other suites depend on that pin. See #1269