Tool: crossref_search (#37)

* Tool: searxng_search

Add searxng_search tool sources and focused docs from develop, sliced as an independent feature. Based on main for minimal diff; marked draft pending scaffold merge.

* Tool: pdf_extract

Add pdf_extract tool sources and docs from develop as an independent feature.

* Tool: wiki_query

Add wiki_query tool sources from develop as an independent feature.

* Tool: openalex_search

Add openalex_search tool sources and docs from develop as an independent feature.

* Tool: crossref_search

Add crossref_search tool sources and docs from develop as an independent feature.

---------

Co-authored-by: aibuddy <aibuddy@dev.hg.fi>

Author: heusalagroupbot

docs/reference/crossref_search.md

@@ -0,0 +1,37 @@
+# Crossref search tool (crossref_search)
+
+Search DOI metadata via Crossref API.
+
+## Usage
+
+```bash
+printf '{"q":"golang","rows":5}' | ./tools/bin/crossref_search | jq
+```
+
+- Required env: `CROSSREF_MAILTO` (used in User-Agent and `mailto` parameter)
+- Optional env: `HTTP_TIMEOUT_MS`
+
+## Input
+
+- `q` (string): search query
+- `rows` (int, default 10, max 50): number of results
+
+## Output
+
+```json
+{
+  "results": [
+    {
+      "title": "...",
+      "doi": "...",
+      "issued": "YYYY[-MM[-DD]]",
+      "container": "Journal ...",
+      "title_short": "..."
+    }
+  ]
+}
+```
+
+## Notes
+- 8s default timeout; up to 5 redirects; SSRF guard blocks private/loopback unless `CROSSREF_ALLOW_LOCAL=1` for tests.
+- On HTTP 429, the tool exits non‑zero and prints a single‑line stderr JSON with `RATE_LIMITED`.