fix(discovery): agentskills.io spec compliance + parity hardening

Mirrors node-commerce — addresses spec violations + parity issues from
end-to-end review against https://agentskills.io/specification:

P0 — spec violations:
- description / license / compatibility / allowed-tools / metadata-values now
  emitted as YAML double-quoted scalars. Fixes the colon / quote / newline
  parse failure the spec explicitly warns about.
- metadata.version emitted as quoted string ("1") not int (1). Spec requires
  metadata values to be strings.

P1 — validation + exports:
- Validate name against the spec regex (1-64 chars, lowercase alphanumeric +
  hyphens, no leading / trailing / consecutive hyphens).
- Validate description length ≤1024 + non-empty.
- Validate compatibility length ≤500.
- Title-block parity fix: tagline + intro now join with blank lines instead
  of being glued together (previously diverged from node output).
- Re-export RailKey + compatible_clients_by_rails from agentscore_commerce
  .discovery (added to __all__).

P2 — additional spec fields:
- Surface optional license, compatibility, allowed_tools frontmatter fields.
- Move homepage from top-level to metadata.homepage (spec-compliant).

P3 — edge cases:
- Drop compatible_clients overrides for rails not in accepted_rails.
- Escape pipe characters in markdown table cells.
- Add /SKILL.md (uppercase) to DEFAULT_DISCOVERY_PATHS for canonical-cased
  alias.

Tests: 49 cases covering spec violations + edge cases + validation throws.
Coverage 95.45% (over 95%).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vvillait88

agentscore_commerce/discovery/__init__.py

@@ -35,11 +35,13 @@
 )
 from agentscore_commerce.discovery.skill_md import (
     BuildSkillMdInput,
+    RailKey,
     SkillMdEndpoint,
     SkillMdIdentityRequirements,
     SkillMdLink,
     SkillMdShippingPolicy,
     build_skill_md,
+    compatible_clients_by_rails,
 )
 from agentscore_commerce.discovery.well_known_mpp import (
     PaymentMethodConfig,
@@ -62,6 +64,7 @@
     "LlmsTxtSection",
     "NoindexNonDiscoveryMiddleware",
     "PaymentMethodConfig",
+    "RailKey",
     "SkillMdEndpoint",
     "SkillMdIdentityRequirements",
     "SkillMdLink",
@@ -77,6 +80,7 @@
     "build_llms_txt",
     "build_skill_md",
     "build_well_known_mpp",
+    "compatible_clients_by_rails",
     "install_flask_noindex",
     "is_discovery_path",
     "is_discovery_probe_request",

agentscore_commerce/discovery/robots_tag.py

@@ -22,6 +22,7 @@
         "/openapi.json",
         "/llms.txt",
         "/skill.md",
+        "/SKILL.md",
         "/.well-known/mpp.json",
         "/.well-known/agent-card.json",
         "/.well-known/ucp",