fix(jsonl-merge): equal-ts entries must converge across machines

[jbetala7]

Fixes #1769

Repro / observed problem

bin/gstack-jsonl-merge is the git merge driver for append-only JSONL state
files (telemetry, learnings, timeline). Its header promises deterministic
resolution: "both appends survive, ordered by wall-clock timestamp where
available, content hash otherwise." That promise breaks when two different
entries share the same ts:

a='{"ts":"2026-05-28T10:00:00Z","event":"a"}'
b='{"ts":"2026-05-28T10:00:00Z","event":"b"}'
printf '%s\n' "$a" > ours; printf '%s\n' "$b" > theirs; : > base
bin/gstack-jsonl-merge base ours theirs && cat ours   # a, b
printf '%s\n' "$b" > ours; printf '%s\n' "$a" > theirs; : > base
bin/gstack-jsonl-merge base ours theirs && cat ours   # b, a  <-- diverges

Root cause

The sort key for a timestamped line was (0, ts) with no further tiebreaker.
Python's sort is stable, so equal-ts entries keep insertion order
(base, ours, theirs). Git assigns the local side to %A ("ours"), so the
same logical merge runs with ours/theirs swapped on the two machines, and
the equal-ts lines come out in opposite order. The merged files diverge and
never converge; the next sync is another conflict, resolved differently again.
gstack-telemetry-log stamps second-granularity timestamps
(date -u +%Y-%m-%dT%H:%M:%SZ), so same-ts collisions are routine. Lines
without a ts were already safe — they order by SHA-256 of the content, which
is side-independent.

Fix

Add the line content as the final sort tiebreaker ((0, ts, line), and
(1, h, line) for the hash path for symmetry). Equal-ts entries now order by
content, identically on every machine. One-line behavioral change to the sort
key.

Testing

New test/jsonl-merge.test.ts (6 tests, bun test):

• equal-ts entries resolve identically with the two sides swapped (the
  convergence regression — fails on main: emits a,b vs b,a; passes
  with the fix)
• no-ts and plain non-JSON lines also resolve side-independently
• exact-duplicate dedup preserved
• timestamped entries still sort ascending by ts
• absent ours/theirs files tolerated (added-file merge)

Verified the convergence test fails on origin/main's driver and passes with
the change. bash -n clean, git diff --check clean.