feat(relations): add multi-target issue relation removal

[ruby-automation]

Summary

• Implements lc issue relation remove ISSUE RELATED_ISSUE... --type TYPE (aliases: r, rm)
• Resolves the exact stored relation by listing the subject issue's relations, matching by wire type + direction + endpoint identifier, then calling issueRelationDelete(id:) — no arbitrary deletion
• blocked-by correctly targets the inbound blocks relation (same reversal logic as add)
• Absent relation is a per-target no-op; ambiguous match (multiple rows for same endpoint pair) fails that target and lists every matching relation ID
• Each target is processed independently; partial failures do not roll back successful deletions

Scope of changes

• app/lib/linear_cli/linear/issue_relation.ex — destroy :destroy action + ManualDestroy module calling issueRelationDelete
• app/lib/linear_cli/linear.ex — delete_issue_relation code interface
• app/lib/linear_cli/cli/commands.ex — issue_relation_remove/1 with per-target result tuples, matching logic, and human/JSON output
• app/lib/linear_cli/cli.ex — remove subcommand spec, dispatch clause, r/rm aliases
• Tests: ManualDestroy unit tests, CLI command tests (17 new scenarios covering all acceptance criteria), alias normalization tests
• documents/ash-domain-erd.adoc — :destroy action and delete_issue_relation interface documented

Test plan

• mix precommit passes (format + credo + tests) — confirmed in commit hook
• 471 tests pass (22 new tests added)
• All acceptance criteria from EXT-42 covered: removal for all types, blocked-by reversal, N+ targets, absent no-op, ambiguous failure, API error, partial failure, human and JSON output, non-zero exit on failure

Closes EXT-42

🤖 Generated with Claude Code

[bougyman]

There's an ugly case at https://github.com/rubyists/linear-cli/pull/254/files#diff-3c0f086fbd3d0b0af6f9c0e48ed5b2fbc473169f2cbdd632bf4e79ee701af2ebR1222 that needs to be idiomatically elixired. (multiple methods with matchers)

[ruby-automation]

Rework: addressed review feedback

Review comment addressed: "There's an ugly case at line 1222 that needs to be idiomatically elixired. (multiple methods with matchers)"

What changed:

Refactored remove_single_relation/4 in app/lib/linear_cli/cli/commands.ex to replace the case matches do block with three pattern-matched do_remove/2 function clauses — the idiomatic Elixir approach:

• do_remove([], related_id) → {:absent, related_id}
• do_remove([relation], related_id) → calls delete_issue_relation, returns {:removed, ...} or {:failed, ...}
• do_remove(relations, related_id) → {:ambiguous, related_id, ids}

The dispatch is now a single pipeline in remove_single_relation/4:

subject_id
|> find_matching_relations(related_id, user_type, all_relations)
|> do_remove(related_id)

No behaviour change. All 471 tests still pass, credo 0 issues (716 mods/funs now with the 3 new do_remove clauses).

Commit: e029548 refactor(relations): use function clauses for remove match dispatch