docs(readme): add oracle_reindex to MCP tool list (5 of 5)

[LanNguyenSi]

Summary

src/mcp-server.ts:138 exports oracle_reindex as the fifth MCP tool, but the README only listed the four read-side tools (oracle_search, oracle_query, oracle_expand, oracle_list_repos) in three places. This PR realigns the headline tool count and gives oracle_reindex a one-sentence gloss.

Closes: agent-tasks README drift audit 2026-05-25 (oracle_reindex MCP tool undocumented).

Changes

• README.md:45 adds oracle_reindex to the in-line tool list and one sentence describing what it does (incremental on-demand reindex, use after merging code you want the oracle to see immediately).
• README.md:80 "the four tools" -> "the five tools" in the docs/mcp.md pointer row.
• README.md:108 adds oracle_reindex to the agent-mode tool slash list.

Reviewer notes (subagent, rigorous)

• Verified 5 server.tool(...) definitions in src/mcp-server.ts (lines 42, 61, 88, 116, 138).
• README sentence is a faithful paraphrase of the tool's own description at src/mcp-server.ts:140.
• docs/mcp.md already documents oracle_reindex (line 35 + usage example at line 47), no follow-up needed there.
• grep -niE 'four tools|four MCP|4 tools' clean in README and docs.

Test plan

• grep '^server\.tool(' src/mcp-server.ts | wc -l returns 5
• grep -niE 'four tools|four MCP|4 tools' README.md docs/mcp.md returns no hits
• Markdown table at line 80 still renders (3 columns, header + 4 data rows)