Add the skills silo, and widen the columns the census broke

[lbesecker195]

Closes #62. Follows #61, which taught the prober to ask for prompts.

The data came first

The measurement that justified this: one remote server in ten offers prompts — 202 servers and 932 prompts across the first 1,957 that answered. My initial sample of eight servers found zero, including servers that declare the prompts capability, and on that basis I nearly did not build this at all. Eight was not enough.

The pages

A skill is an MCP prompt: something the user invokes deliberately, as against a tool, which the model calls on its own. Every page leads with that, because it is the part a reader arriving from a search has usually not understood.

• /servers/:ns/:name/skills — index, only where a listing has prompts
• /servers/:ns/:name/skills/:skill — one skill
• /servers/:ns/:name/skills/:skill/:client — H1 AGENT SERVER Skill/SKILL, H2 How to: AGENT SERVER SKILL

Each client page says where the prompt actually surfaces in that client — a slash command in Claude Code, the attachment menu in Claude Desktop — because sending a reader to look for a menu their client does not have is the difference between a page that works and one that wastes their time.

prompts is deliberately not castable from user input. The pages state these names were read from the server itself; a publisher able to submit them would make that claim false.

Two bugs the census exposed

1. {:array, :string} is varchar(255)[]. Resource URIs pass 255 characters routinely. The write raised Postgrex.Error out of the runner's Task.async_stream and killed the batch around it — one long URI cost the other 399 probes in its batch, which is what stalled the first census run. Columns widened to text[], and record/3 now rescues so one bad row can never be fatal to the rest.
2. 1..0 descends in Elixir (step -1), so page in 1..0 was true and /sitemaps/tools-1.xml served an empty 200 instead of a 404 whenever nothing had tools. Pre-existing; fixed for both files.

Tests: 163 passing (+17).

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Pages affected:

• MCP Harbor -- registry home, search and newly added servers.
• MCP server directory -- the catalogue the skill pages hang under.

🤖 Generated with Claude Code