# Federation: Agent-Driven Hash Discovery

## Context

Content-addressed fetch shipped. Every FETCH response includes `content-hash`, servers maintain in-memory hash indexes, and `FETCH /sha256-<64hex>` works. The hub pattern is live at `mark://hub.demarkus.io`. This plan adds two MCP tools implementing the agent-driven discovery pattern from SPEC.md Section 12.1.

## What Shipped

Two new MCP tools + one new package. Zero server changes. Zero new verbs.

### `client/internal/index` package

Parse, Build, and Merge functions for markdown hash index documents. Format:

```markdown
# Content Index

> Source: mark://docs.example.com
> Indexed: 2026-03-07T15:30:00Z
> Documents: 42

| Hash | Server | Path |
|------|--------|------|
| sha256-<64hex> | mark://docs.example.com | /guide.md |
```

### `mark_resolve` MCP tool

Resolves content by hash using a hub index document. Validates hash format, fetches the index, finds matching servers, tries each one, verifies `content-hash` in response matches.

### `mark_index` MCP tool

Crawls a source server (LIST + FETCH), collects `content-hash` from every document, publishes a hash index to a target hub.

**Safety features:**
- **Manifest check enforced by tool** — target must have `/.well-known/agent-manifest.md` or tool refuses to publish
- **`force=true`** — explicit override, visible in MCP approval
- **`dry_run`** — returns index without publishing
- **Source manifest warning** — warns if source has no manifest (but proceeds)
- **1000 doc cap** — prevents runaway crawls

### Files

| File | Change |
|------|--------|
| `client/internal/index/index.go` | NEW: Parse, Build, Merge |
| `client/internal/index/index_test.go` | NEW: table-driven tests |
| `client/cmd/demarkus-mcp/main.go` | mark_index and mark_resolve tools |
| `client/cmd/demarkus-mcp/main_test.go` | Tests for both tools |

## Key Design Decisions

- No server changes — federation is agent-layer concern
- Index is a markdown document — published via PUBLISH, fetched via FETCH
- Manifest check is tool-enforced — agent can't bypass without explicit `force`
- Hash verification on resolve — defense in depth against untrusted servers
- Merge preserves multi-source indexes — single hub can aggregate many servers