feat(create): --json returns the created issue (#64)

[rush-cloud-agents]

Closes #64.

linear create --json prints one JSON object for the created issue: id, identifier, title, url, status, priority, plus reminder when --remind-at was given (null if it could not be set). Nothing else goes to stdout; warnings and the "applied" context stay on stderr. A rejected create exits 1.

With --from-file, --json prints a JSON array with one entry per input line: the created issue, or {"error": ..., "title": ...} for a failure (a non-object line becomes an error entry). Without --json, output is unchanged. README, skill.md and CHANGELOG list the flag.

Written by Claude Code in a Rush Cloud run (commits aa95572, d3a76eb). A separate OpenCode run on the same issue could not push its branch and replaced this description with its own; this restores it to match the code.

$ python3 test_linear.py
Ran 239 tests in 1.232s
OK (skipped=4)

[rush-cloud-agents]

Non-author review (subagent): NON-BLOCKING, conformant to #64, stdout clean on all traced paths. Two SHOULD findings (exit 0 with empty stdout on a rejected single create; non-object JSONL line crashing bulk mode) fixed in the follow-up commit with tests.