Replace scoped resources/list enumeration with skill://index.json

Enumeration now uses a well-known skill://index.json resource whose
format mirrors the Agent Skills .well-known discovery index: same
$schema, same skills[] shape. Two deltas from the HTTP index: url
holds the full skill:// URI, and digest is omitted (transport handles
integrity over an authenticated connection). type MUST be "skill-md"
since archives don't apply when every file is individually
addressable.

This drops the SEP-2093 dependency entirely — the extension now has
zero protocol dependencies beyond resources/read.

Also includes review feedback from PR #69:
- Security: skill content MUST be treated as untrusted input; hosts
  MUST NOT honor local-execution mechanisms (hooks, scripts) without
  explicit opt-in; "more harm" softened to "as much harm"; user
  inspection SHOULD be supported
- Cite agentskills.io parent-directory rule for the final-segment
  constraint
- No-nesting constraint reworded per cliffhall suggestion
- Abstract mentions progressive disclosure as delegated concern
- Hosts section: SHOULD load frontmatter into context, SHOULD surface
  skills for user enable/disable
- read_resource signature marked illustrative
- "community has explicitly pushed back" softened

Author: pja-ant