* feat(ci): split backend unit tests into parallel CI shards
Split the single offline backend `make test` job into four GitHub Actions
matrix shards (SPLITS=4, GROUP=1..4) via pytest-split, so the ~12k-test suite
runs in parallel instead of in one 15-minute job. Each shard runs on its own
runner with its own Postgres/Redis services; fail-fast: false lets a failing
shard report its owned tests without cancelling its peers.
`make test` stays the canonical full-suite entry point; CI now calls the new
`make test-shard SPLITS=4 GROUP=N`. tests/blocking_io remains owned solely by
the dedicated blocking-I/O workflow (excluded via --ignore), extending #5105.
Fixes#5088
* test(ci): make backend test shards duration-aware and pin the contract
Make `make test-shard` an explicit least_duration split that READS
backend/.test_durations (read-only for shards, so concurrent CI jobs never
race writes on it), and add `make test-shard-durations` to regenerate that
file from the full offline suite. Update the CI unit-test workflow contract to
call `make test-shard SPLITS=4 GROUP=<n>` and assert the shard command carries
--splits 4, --group 2, -m "not live", --ignore=tests/blocking_io and
--splitting-algorithm least_duration. Verified on the real 13,140-test normal
suite that the four shards are pairwise disjoint and their union equals the
unsplit suite.
Refs #5088
* test(ci): fail fast when the duration baseline is missing
`make test-shard` now requires backend/.test_durations and exits with a clear
error instead of letting pytest-split silently degrade to an even (count-based)
split. Harden the CI contract test to pin `--durations-path=.test_durations` and
to assert the repo ships the committed duration baseline.
Refs #5088
* docs: trim backend/AGENTS.md within guidance budget
* test(ci): add backend test duration baseline
Add the duration baseline generated by a full offline backend run on a
GitHub-hosted ubuntu-latest runner (the same runner type the shards use), so
`make test-shard` balances the four matrix shards by real wall-clock cost.
Refs #5088
* test(ci): make the duration writer honor DURATIONS_FILE
`test-shard-durations` now writes `--durations-path=$(DURATIONS_FILE)` instead of a
hard-coded .test_durations, so the reader and writer stay consistent when the
path is overridden.
Refs #5088
* test(ci): address sharding review feedback
* test: isolate subagent execution capacity state
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
