Error catalogue: every error apic prints, its exit code and the fix #65

Why

apic's errors are already good ("it is captured by request login; run apic run login first"). A single page that lists every one, with the exit code and what to do, is what the FAQ, the agent guide and the VS Code extension's quick fixes all want to link to, and it is the page search engines land people on.

What

• Give every user-facing error a stable code (E101 missing variable, E201 unknown request, E301 transport, …) carried on the error type in internal/runner/errors.go and surfaced in --json as "error": {"code", "message", "hint"} (additive to the current shape; README contract note).
• docs/errors.md generated by task docs:errors from a table in the code (code, exit status, message template, hint, longer explanation), so it cannot drift; CI staleness check like the other generated docs.
• Each printed error ends with the short URL of its entry when stderr is a terminal (see https://datagriff.github.io/api-caller/errors/#e101), and apic validate diagnostics carry the same codes.
• The FAQ links each answer to its code; docs/agents.md tells agents to read error.code.

Where

internal/runner/errors.go, internal/cli/root.go (error printing), internal/output, scripts/errors/, docs/errors.md, mkdocs.yml (Reference nav), docs/faq.md, docs/agents.md, docs/cli.md.

Tests

Every code in the table is produced by at least one test (a test that walks the table and checks a registry of "exercised" codes); --json error shape.