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.
• Confirm filtered mutations, support --yes/--dry-run, and process one page of up to 100 matches with a truncation warning.
• Skip already-unassigned issues in filter mode so repeated runs reach later assigned issues.
• Expose the capped first-page read through an Ash generic action and keep the README and Ash ERD current.

Verification

• mise exec -- mix setup
• mise exec -- mix ci (root 52 tests; app 596 tests including one doctest)
• mise exec -- mix test test/linear_cli/cli/commands/issues/mutations_test.exs from app/ (79 passed)
• Hosted PR checks pass for the latest commit.

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.

[ruby-automation]

Rework for the latest code-review findings is pushed in 981cf86.

• Filter-only unassignment now passes assigned_only: true; with --no-mine and no explicit assignee, the API filter includes assignee: {null: false} so reruns skip already-unassigned issues. Ordinary issue list --no-mine still sends no assignee clause.
• Replaced the plain Linear.issues_first_page/1 domain helper with the Ash Issue.list_first_page generic action and issues_first_page code interface.
• Removed the unused fetch_all_pages/:infinity path and unreachable name-based assignee filter. Updated the README, ERD, and regression tests.
• mise exec -- mix ci passes locally (root 52 tests; app 596 tests including one doctest). Hosted PR checks for 981cf86 pass; the Burrito regression remains skipped because the change does not affect Burrito inputs.

The JSON prompt and workspace-wide lookup follow-ups remain out of scope for this narrow rework. No new GitHub review comments are present.

[ruby-automation]

Follow-up to the rework summary: 35ebee8 adds the exact regression invocation requested by review, issue unassign --no-profile --no-mine --team ENG --yes, and asserts the outgoing assignee: {null: false} clause. The preserved issue list --no-mine assertion remains in the domain tests.

The commit hook and final mise exec -- mix ci pass (root 52 tests; app 596 tests including one doctest). Hosted checks for 35ebee8 also pass, with the unrelated Burrito regression job skipped. No GitHub review objects are present.