fix(linear): use String teamId for label queries

[camwest]

Summary

Fix Linear label queries by changing teamId variable type to String! for team(id: $teamId) lookups.

This unblocks these Linear flows:

• isq label list
• isq issue create --title "..." --label ...
• isq issue label <id> add <label>

Root Cause

Two queries used query($teamId: ID!) while querying team(id: $teamId). Linear validates this and returns:

Variable "$teamId" of type "ID!" used in position expecting type "String!"

Changes

• Added TEAM_LABELS_QUERY in src/forges/linear/issues.rs with query($teamId: String!)
• Added LIST_LABELS_QUERY in src/forges/linear/forge_impl.rs with query($teamId: String!)
• Updated get_label_ids and list_labels to use the shared constants
• Added regression tests:
  • forges::linear::issues::tests::team_labels_query_uses_string_team_id
  • forges::linear::forge_impl::tests::list_labels_query_uses_string_team_id

Validation

• cargo test (191 passed)

Closes #159

[camwest]

• fix(linear): use String teamId for label queries #160 👈 (View in Graphite)
• master

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