feat(labeler): add /assign and /unassign support

[amanbn8]

Fixes #109

What

Adds /assign and /unassign slash-command support to utilities/labeler via two new action kinds, assign and unassign, usable from match rules:

- name: assign
  kind: match
  spec:
    command: "/assign"
  actions:
  - kind: assign

- name: unassign
  kind: match
  spec:
    command: "/unassign"
  actions:
  - kind: unassign

Comment	Effect
/assign	assigns the comment author
/assign @alice bob	assigns alice and bob
/unassign @alice	removes alice

How

• GitHubClient gains AddAssignees, RemoveAssignees and IsAssignee (all exist in go-github v55; no new dependencies).
• LabelRequest gains an optional CommentAuthor. It is wired through as an optional trailing comment_author CLI argument and an optional comment_author input on .github/actions/labeler-action (pass ${{ github.event.comment.user.login }}). Existing callers that omit it keep working; only bare /assign / /unassign need it.
• Logins are normalized: leading @ stripped, case-insensitive de-dupe. Already-assigned users are skipped on assign, not-assigned users on unassign. Users who are not assignable in the repo (per IsAssignee) are skipped with a log line rather than failing the whole run, so one bad login does not block the rest.
• Match-rule commands must now be the first whitespace-delimited token of a line, so /assign no longer also fires on /assignee. Leading whitespace is still tolerated (strings.Fields). This matches the direction of Update labeler #640.
• README documents the new actions and CLI argument.

Testing

cd utilities/labeler
gofmt -l .        # only the pre-existing comprehensive_test.go
go vet ./...
go test -v        # all pass, incl. 11 new tests in assign_test.go
.github/scripts/check-go-version.sh   # All Go versions match 1.27.1

New tests cover explicit users, bare self-assign, no-author no-op, skipping already-assigned/duplicate/unassignable users, IsAssignee API error, unassign explicit/self/not-assigned, and that /assignee does not trigger /assign.

Not tested against a live repository; the GitHub calls are the straightforward go-github wrappers.

Notes

The mock GetIssue now reflects the mock's assignee state. Kept the change additive to limit conflicts with #640; the processMatchRule token check is the one overlapping hunk.

[github-actions]

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[Copilot]

🟡 Changes recommended

Malformed targets can unexpectedly select the comment author or prevent valid assignments in the same command.

2 open issues

• Malformed @​ target incorrectly falls back to comment author · New
• Malformed login prevents valid assignments from being applied · New

What changed in this PR

Adds configurable /assign and /unassign commands to the labeler, addressing #109.

Changes:

• Adds assignee actions with login normalization and eligibility checks.
• Passes an optional comment author through the CLI and Docker action.
• Tightens command matching and adds documentation and tests.

File	Description
utilities/​labeler/​README.md	Documents assignment commands and author input.
utilities/​labeler/​main.go	Accepts the optional comment-author argument.
utilities/​labeler/​labeler.go	Implements assignee actions and exact-token matching.
utilities/​labeler/​labeler_test.go	Extends mocks with assignee state and operations.
utilities/​labeler/​assign_test.go	Tests assignment behavior and command matching.
.github/​actions/​labeler-action/​action.yml	Forwards the optional comment-author input.

_{Review effort: Balanced}