feat: make online test team configurable via LINEAR_TEST_TEAM_KEY

[lightstrike]

Summary

• Adds LINEAR_TEST_TEAM_KEY environment variable support to all online test files
• When set, tests look up the team by key instead of blindly using teams().first(1) (which picks the first team alphabetically — fragile across workspaces)
• When unset, behavior is unchanged (falls back to first(1))
• Consolidates ~20 inline team-lookup patterns in CLI tests into a shared test_team_key() helper, reducing duplication

Closes #105

Files changed

• crates/lineark-sdk/tests/online.rs — added async test_team_id() helper, replaced 8 inline lookups
• crates/lineark-sdk/tests/blocking_online.rs — added sync test_team_id_blocking() helper, replaced 3 inline lookups
• crates/lineark/tests/online.rs — added test_team_key() and test_team_id_cli() helpers, replaced ~20 inline lookups

Test plan

• cargo build --workspace compiles cleanly
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --check passes
• All offline tests pass (76 CLI + 44 SDK + 38 codegen)
• SDK online tests pass with LINEAR_TEST_TEAM_KEY=STA (34/34)
• Blocking online tests pass with LINEAR_TEST_TEAM_KEY=STA (11/11)
• CLI online tests pass with LINEAR_TEST_TEAM_KEY=STA (47/49 — 2 pre-existing ProjectCreate UUID collision failures unrelated to this change)
• Without env var set, falls back to original first(1) behavior

[flipbit03]

Thanks for putting this together — it's solid work and the helper consolidation is a nice cleanup.

However, we've decided to go a different direction on this. Rather than making the team configurable, we want to fix the root cause: tests should not depend on pre-existing teams at all. The TeamGuard RAII pattern already exists and is proven in the team CRUD tests — the fix is to migrate all teams[0] / teams().first(1) call sites to create their own team, making every test fully self-contained (which is already the stated policy in CLAUDE.md).

See #105 (now closed) for the full rationale, and #117 for the tracking issue.

Closing this PR without merging — but thanks again for surfacing the problem and for the effort here!