feat: support filter-only batch issue unassignment

[ruby-automation]

Summary

• add filter-only issue unassign with assignee, team, project, state, status, and label selectors
• resolve profile/project defaults safely and reject empty or mixed ID/filter requests
• fetch every filtered issue page before applying ordered concurrent updates
• document no-match output, partial-update behavior, and the intentional --unassigned exclusion

Verification

• mise exec -- mix ci
• mise exec -- mix test test/linear_cli/cli/commands/issues/mutations_test.exs test/linear_cli/cli/commands/issues/read_test.exs test/linear_cli/cli/profile_defaults_test.exs test/linear_cli/linear/paginate_test.exs (133 passed)
• mise exec -- mix test from app/ (589 tests and 1 doctest passed)

Linear issue: EXT-68

[ruby-automation]

Rework for the implementation-review feedback is pushed in 8902fd1.

• Filtered unassignment now confirms with Unassign N issue(s)?; -y/--yes bypasses the prompt and --dry-run lists matches without mutations.
• Strict project resolution keeps exact matches non-interactive and prompts for partial matches; no-match input still fails before the issue query.
• Assignee filters resolve exact names/display names without prompting, prompt for partial matches, and send an assignee.id filter.
• The README, Ash ERD, and module-structure documentation describe the new scope and controls; redundant empty-list handling was removed.
• mise exec -- mix ci passes locally (root 52 tests; app 595 tests including one doctest), and PR CI for 8902fd1 is green. No new GitHub review comments were present after the rework push.

[ruby-automation]

Rework for the reporter's latest implementation-review direction is pushed in a715d5f.

• Filter-only unassignment now requests one 100-record issue page, never follows the cursor, and warns when hasNextPage is true that only the first 100 will be processed.
• JSON output remains valid because the truncation warning goes to stderr.
• Added regression coverage for the single-page request, 100-match cap, warning, first-page errors, and the pagination helper; updated the README and Ash ERD.
• mise exec -- mix ci passes locally (root 52 tests; app 597 tests including one doctest), and hosted PR checks for a715d5f pass. No new GitHub review comments are present.