fix(config): propagate --jq to nested pup calls via PUP_FILTER env

[jkirsteins]

Problem

--jq is silently ignored when an extension delegates its HTTP call to a nested pup process, even though -o/--output works in the exact same chain. When an extension subcommand shells out through a helper to a nested pup api call, -o table propagates and takes effect but --jq does not:

pup --jq '.[] | {id: .id, service: .service_name, admin_teams: .admin_teams}' -o table test-extension list-items

Before this fix — the jq projection is dropped; every column comes back (data anonymized):

+------+--------+----------------------+----------------------+-------------------+-------------------+------------+---------------+--------+--------------+-------------------+-------------------+
| id   | name   | created_at           | updated_at           | created_by        | updated_by        | test_drive | route_id      | domain | service_name | service_namespace | server_group_name |
+==================================================================================================================================================================================================+
| 587  |        | 2026-04-20T20:16:35Z | 2026-04-20T20:16:35Z | agent@example.com | agent@example.com | false      | Rk3Lm9Qp2Xz=  | demo   | edge-backend | platform          | probe             |
|------+--------+----------------------+----------------------+-------------------+-------------------+------------+---------------+--------+--------------+-------------------+-------------------|
| 588  | sample | 2026-04-20T20:16:35Z | 2026-06-24T19:48:17Z | agent@example.com | agent@example.com | false      | 8Hn1Zu7BsfLa= | demo   | edge-backend | platform          | probe             |
|------+--------+----------------------+----------------------+-------------------+-------------------+------------+---------------+--------+--------------+-------------------+-------------------|
| 1104 |        | 2026-05-13T00:49:57Z | 2026-05-13T00:49:57Z | agent@example.com | agent@example.com | false      | eJ5yHpxLNCPc= | demo   | gateway      | commerce          | sandbox           |
|------+--------+----------------------+----------------------+-------------------+-------------------+------------+---------------+--------+--------------+-------------------+-------------------|
| 1    |        | 2026-01-15T09:00:00Z | 2026-01-15T09:00:00Z | agent@example.com | agent@example.com | false      | aB2cnoP8qrS9= | demo   | search-api   | search            | search            |
+------+--------+----------------------+----------------------+-------------------+-------------------+------------+---------------+--------+--------------+-------------------+-------------------+

After this fix — the jq projection is applied before formatting:

+------+--------------+-------------+
| id   | service      | admin_teams |
+===================================+
| 587  | edge-backend | [ops]       |
|------+--------------+-------------|
| 588  | edge-backend | [ops]       |
|------+--------------+-------------|
| 1104 | gateway      | [frontend]  |
|------+--------------+-------------|
| 1    | search-api   | [search]    |
+------+--------------+-------------+

Root cause

Config::from_env resolves output_format with an env fallback (DD_OUTPUT / PUP_OUTPUT), which is why a nested pup process inherits --output. jq had no equivalent fallback — it was hardcoded to None and only ever set from the literal --jq CLI flag. PUP_FILTER is already injected into every extension's environment (src/extensions/exec.rs) for exactly this purpose, but nothing read it back.

Fix

One line in src/config.rs — give jq the same env fallback output_format already has:

jq: env_or("PUP_FILTER", None),

Precedence is preserved: an explicit --jq flag still overrides the env (main.rs applies it after Config::from_env). Top-level pup behavior is unchanged when PUP_FILTER is unset; a manually-exported PUP_FILTER is now honored, matching the existing unguarded PUP_OUTPUT behavior.

Testing

• Extended config::tests::test_pup_env_inherited_by_child to cover PUP_FILTER (asserts None when unset, Some(expr) when set).
• cargo fmt --check, cargo clippy -- -D warnings, cargo build --release all clean.
• Manually verified the repro end-to-end with the patched binary on PATH.

Docs

Added PUP_FILTER to the env-var table in docs/EXTENSIONS.md and extended the "read back by a child pup process" note.