firecrawl-research-papers skill documents a non-existent --k flag (should be --limit)

The `firecrawl-research-papers` skill documents `--k ` for the `firecrawl research search-papers` and `related-papers` CLI commands, but the CLI has no `--k` flag. The result-count flag is `--limit`. Because the CLI silently ignores unknown options, following the skill literally is a silent no-op — users get the default count (40) instead of the value they pass.

**Affected file:** `skills/firecrawl-research-papers/SKILL.md`

Two lines:
- `CLI: firecrawl research search-papers [--k ]`
- `CLI: firecrawl research related-papers --intent [--mode ] [--k ]`

**Actual CLI (firecrawl-cli@1.19.18):**
```
$ firecrawl research search-papers --help
--limit Number of results to return (default: 40)
-k, --api-key Firecrawl API key (overrides global --api-key)
```
There is no `--k` long option — `-k` is `--api-key` (a key string, not a count).

**Repro — `--k` is silently ignored:**
```
$ firecrawl research search-papers "test query" --k 3
## [pmid:...] ... # returns results at the default limit (40), exit 0 — not capped to 3
```

**Suggested fix:** replace `--k ` with `--limit ` in both CLI examples. (The MCP signatures `...(query, k?)` are a separate surface and may be correct for the MCP tool — only the CLI lines are affected.)

Open skills/firecrawl-research-papers/SKILL.md and inspect the two CLI examples for search-papers and related-papers. Confirm the CLI help output uses --limit for result counts, then update both examples; the MCP signatures should remain unchanged. Done means both CLI examples document --limit and no longer use the non-existent --k count flag.

Written by the indexing model from the issue text.