fix(kyberforge): tell an unparsable Contributing-files block from an explicit (none)

parse_contributing_files documented that callers depend on None vs [], because a
parse failure returning [] would silently disable the check. Only check 8
honoured it; checks 4/5 (skill-audit) and 3/4 (agent-audit) used a truthiness
test, so an unreadable block disabled them without a word.

Two live corpus entries were skipping this way. A sweep of all 32 sources.md
found 134 entries, exactly 2 parsing to None, both in gitea-files: one heading
carried an inline parenthetical that defeated both regexes, and one (none) was
written without its leading bullet. Also pins EMPTY_SOURCE_KEYS_RE to the two
indents parse_source_keys actually reads.

agent-audit had no INFO tier at all, so it gains one rather than reporting a
check that could not run as a FAIL.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EJJrm5YmacbwMdzZpXcoti

plugins/gitea/skills/gitea-files/references/sources.md

@@ -2,13 +2,13 @@
 
 ## gitea-mcp-repo
 
-**Description:** Official gitea-mcp repository (v1.3.0); operation/*.go source files documenting all 55 MCP tools, their parameters, and CLI flags.
+**Description:** Official gitea-mcp repository (v1.3.0); operation/*.go source files documenting all 55 MCP tools, their parameters, and CLI flags. Tool parameters and SHA/concurrency behavior were cross-checked live against the deployed MCP tool schemas via `ToolSearch`, per this repo's process for resolving schema-vs-docs drift, rather than copied from the derived research doc.
 
 **Source:** https://gitea.com/gitea/gitea-mcp
 
 - **Research doc:** plugins/gitea/docs/research/docs/gitea/sources.md
 
-**Contributing files:** (tool parameters and SHA/concurrency behavior cross-checked live against the deployed MCP tool schemas via ToolSearch)
+**Contributing files:**
 - SKILL.md (Gotchas — cross-flow parameter and encoding traps; Dispatch)
 - references/reading.md (read-tool parameters, `ref`/`tree_sha` selection, tree pagination)
 - references/writing.md (write-tool parameters, SHA/concurrency behavior, canonical call sequences, failed-write triage)
@@ -45,4 +45,4 @@
 
 - **Research doc:** plugins/gitea/docs/research/docs/gitea/sources.md
 
-**Contributing files:** (none)
+- **Contributing files:** (none)