http: classify missing token, invalid token, and insufficient scope distinctly

## Problem
Authentication and authorization failures currently collapse into generic HTTP errors even though missing credentials, invalid/expired credentials, and insufficient scopes require different remediation. That weakens machine-readable diagnostics for hosts and operators.

## Why now
This server sits directly on GitHub token-policy boundaries. Clients and wrappers need explicit failure classification to decide whether to prompt for auth, refresh a token, or request additional scopes.

## Expected behavior
Missing token, invalid/expired token, and insufficient scope paths should each expose a distinct machine-readable error code while preserving the existing auth challenge semantics.

## Claim-to-codepath map
- `pkg/http/middleware/token.go`
- `pkg/http/middleware/pat_scope.go`
- `pkg/http/middleware/scope_challenge.go`
- `pkg/errors/error.go`
- `docs/error-handling.md`

## Evidence packet
- Commit under test: `fa87e4fc9e4cd1f514cf4c236b9135987a328512`
- Runtime: Go HTTP middleware tests on macOS
- Repro concept: exercise requests with no token, malformed/invalid token, and OAuth requests lacking required scopes.

## Validation requirements
- Add targeted middleware/error tests for each classification.
- Preserve `WWW-Authenticate` challenge behavior where applicable.
- Document the new machine-readable codes.