PR #1744 fixed literal `\n\n` rendering for the CLI surfaces (`issue
create / update --description`, `issue comment add --content`) but the
agent-completion path bypasses the CLI entirely: the daemon POSTs the
agent's stdout to `/api/daemon/tasks/:id/complete`, and `TaskService.
CompleteTask` writes `payload.Output` straight into `createAgentComment`
and `CreateChatMessage` without decoding. Models (e.g. Codex) routinely
emit Python/JSON-style `\n` literals in their final output, which then
land in the DB as the 4-char escape sequence and render as one wall of
text in the issue/chat panel — exactly the bug report in #1820.
- Move `unescapeFlagText` from `server/cmd/multica/cmd_issue.go` to
`server/internal/util/text.go` as `UnescapeBackslashEscapes` so the
CLI and the service layer share one implementation. The full
contract-boundary test suite moves with it.
- Apply `UnescapeBackslashEscapes` to `payload.Output` before it
reaches `createAgentComment` and `CreateChatMessage` in
`TaskService.CompleteTask`. Same `\n / \r / \t / \\` decoding as the
CLI; other escape sequences (`\d`, `\w`, `\u`, etc.) pass through
verbatim so regex/format strings in agent output survive.
Closes#1820
