feat: issues batch-update for bulk issue updates

[lightstrike]

Summary

• Enhance codegen emit_mutations.rs to detect list entity fields and generate Result<Vec<T>> return types
• Add execute_batch_mutation to SDK Client for mutations returning entity arrays
• Add issueBatchUpdate mutation to operations.toml and regenerate SDK
• New CLI subcommand: lineark issues batch-update ID1 [ID2...] --status/--priority/--assignee/--labels/--project/--cycle
• Applies the same IssueUpdateInput to multiple issues at once, returns updated issue table
• Update lineark usage reference

Test plan

• cargo fmt --check — clean
• cargo clippy --workspace -- -D warnings — clean
• cargo test --workspace — all offline tests pass (81)
• CLI online tests pass: --priority, --status "Done", invalid ID error path
• Manual e2e: batch update with priority and status verified
• SDK online tests blocked by feat: make test team configurable via environment variable #105 — pre-existing test panics and aborts the test_with runner before new tests execute

Blocker: #105 must be resolved for the full SDK online test suite to run.

[flipbit03]

Hey @lightstrike, thanks for working on this!

We've been looking at the approach here and think there's a simpler path that avoids the codegen duplication and the new execute_batch_mutation method entirely.

The key insight is that execute_mutation<T> already works for list entity fields — if T = Vec<Issue> and entity_field = "issues", then payload.get("issues") returns a JSON array, and serde_json::from_value::<Vec<Issue>>() deserializes it correctly. No special client method needed.

What's needed instead:

1. Blanket impl GraphQLFields for Vec<T> (same pattern as fix: blanket impl GraphQLFields for Option<T> for nullable queries #121 did for Option<T>) — just 3 lines in field_selection.rs:

   impl<T: GraphQLFields> GraphQLFields for Vec<T> {
       type FullType = T::FullType;
       fn selection() -> String { T::selection() }
   }

   This works because the GraphQL selection set is identical whether requesting a single entity or a list — only the JSON response shape differs.

2. Minimal codegen tweak in emit_mutations.rs: when the entity field is a list type, generate Result<Vec<T>, LinearError> as the return type and call self.execute_mutation::<Vec<T>>(...) instead of self.execute_mutation::<T>(...). Same method, different type parameter. No need for duplicated if/else code paths.

We have push access to your branch, so we'll push a simplified version that takes this approach. The mutation itself (issueBatchUpdate) and the CLI integration stay — just the plumbing gets simpler.