Adds `spacedock-release journey-delta`, which selects the latest-by-captured_at
observation per scenario/model from the previously published release ledger,
diffs it against the current PR run's freshly emitted records, and posts the
result as a single sticky-updating PR comment (gh pr comment --edit-last).

Wires a new journey-delta-comment job into runtime-live-e2e.yml, gated to
pull_request events with its own pull-requests: write permission.

Adds two spacedock-release CLI primitives the AC-4 backfill runbook composes:
shallow-boot-window-record extracts a shallow-boot-window observation from an
archived claude-stream.jsonl (reusing AC-1's extraction logic, hoisted out of
its test file into internal/ensigncycle so a non-test binary can import it),
and ledger-diff proves a rebuilt journey-costs ledger equals the backed-up
original plus exactly the named added scenarios before any `gh release upload
--clobber` runs.

Both subcommands get CLI-level tests (arg parsing, file I/O, error paths)
matching this file's existing per-subcommand test convention, on top of the
already-covered package-level logic (BuildShallowBootWindowRecord,
DiffAddedScenarios). Fixes a dangling main.go doc-comment reference to a
scripts/backfill-shallow-boot-window.sh that was never created.

Does not run the live gh release upload or the actual 3-release backfill —
that stays an explicit captain-flagged manual step.

Validation's detached adversarial audit found that reverting the "Download
latest journey metrics artifacts" step's per-run cp target back to the flat-
copy overwrite bug (the exact collision AC-2 fixes) left the full test suite
green — the Go-side consumer (AggregateLedger/ReadRecordsDir) was fixture-
tested, but the shell script producing the layout never was.

Extends internal/release/journey_workflow_test.go's existing extractStepRun +
stubbed-gh harness with a positive-path test (two stubbed runs, same record
filename, different content, asserts both land in their own subdirectory
intact) and an adversarial twin that reverts the cp target and asserts the
two runs collapse into one file — proving the positive test actually
discriminates a fixed script from a broken one.

… steps

Validation's cycle-3 adversarial audit found that reverting ONLY the "Post
the journey-cost delta PR comment" step's --metrics-dir back to the old
broken hardcoded path left the full suite green — nothing verified the Post
step stayed wired to the "Locate this run's journey metrics" step's actual
output directory. Same class of gap Cycle 1 found: a step's own mechanism
tested in isolation, the wiring between two steps not.

Adds TestJourneyDeltaLocateAndPostStepsShareMetricsDir, which extracts both
steps' real run: blocks and derives each step's directory value from the
actual script text (not a hardcoded assumption), then asserts they match.
Verified it catches the exact adversarial mutation validation performed.

Also folds in the two minors from the same review: fixes a stale "backfill
CLI" doc-comment reference in shallow_boot_window_record.go left over from
the AC-4 TRIM (should read "runbook"), and adds
TestRenderJourneyDeltaCommentShowsTokenClassBreakdown, which checks the PR
comment's Cache Read Δ / Cache Creation Δ table cells land in the correct
(non-swapped) columns — verified it catches a reversion to the old
Turns/Tokens-total/Cost table shape.

…-metrics-record-and-pr-delta

# Conflicts:
#	cmd/spacedock-release/main.go