docs(readme): add codegraph_explore to MCP Tools table and bump Node badge to 20+

[AimoreRRD]

Two small README mismatches between the docs and the current project state:

1. codegraph_explore is missing from the MCP Tools table

src/mcp/tools.ts exports 9 tools, but the table near the bottom of the README lists only 8 — codegraph_explore is absent. The tool is referenced prominently elsewhere in the same README (lines 96, 239, 243, 246, 247) including being singled out as a primary exploration tool, so a reader who skims the table can miss that it exists.

Added a row between codegraph_node and codegraph_files to match the source-file order. Description matches the opening sentence of the tool's own description field in src/mcp/tools.ts.

2. Node.js badge is stale (says 18+, should be 20+)

package.json engines has required >=20.0.0 since commit 9b06b0e (fix(db): require better-sqlite3 ^12.4.1 so Node 24 gets the native backend), which explicitly raised the floor because Node 18 was EOL and dropped from better-sqlite3 12.x prebuilds. The badge still claims "Node.js 18+", which misleads users.

Bumped the badge to 20+. (Left the upper bound alone — the engines pin <25.0.0 is being addressed by PR #222 and a one-shot badge change shouldn't pre-empt that.)

Scope

• Only README.md touched.
• 1 line added, 1 line changed.
• No code, no tests, no behaviour change.

[colbymchenry]

Merged — thanks @AimoreRRD! 🎉 Took Part 1 (the codegraph_explore row): good catch, tools.ts exports 9 tools but the table listed 8. I added it to the full 'MCP server exposes these tools' table only — not the curated 'lightweight tools the main session may use directly' table above it, since explore is intentionally the heavy/delegate-it tool there.

Dropped Part 2 (Node 18+→20+ badge): that badge no longer exists — it was replaced by 'Node.js bundled · none required' when CodeGraph became self-contained (bundles its own runtime), so there's no user-facing version floor to advertise anymore. Credited to you via squash. 🙏