Unpin the Skills sample links and consolidate the frontmatter-verification docs - #1

Thanks for carrying modelcontextprotocol#1864 over, @girishkvs. I'm happy to consolidate on modelcontextprotocol#1856 and will close modelcontextprotocol#1864 in its favour. This PR targets your skills_sep2640 branch with two small follow-ups.

What I checked

• The import is complete. f3d028b7's tree (ba9234da) is identical to the Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills modelcontextprotocol/csharp-sdk#1864 head ecb11e62, so nothing was dropped or rewritten.
• Your fixes in 6b0870cb are right.
  • skills/get cache hints: the stable spec does make ttlMs/cacheScope required on GetSkillResult, and Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills modelcontextprotocol/csharp-sdk#1864 missed it.
  • YAML reader: your four new test files fail 30 of 67 cases against the Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills modelcontextprotocol/csharp-sdk#1864 head. A sample of the expected values (next-line scalars, tab-led folded lines, unseparated # comments after quoted and flow values, |2 indentation indicators, flow-key classification) agrees with the yaml npm parser (2.5.1, core schema).
  • All 470 Skills tests pass on your head on net10.0.

What this PR changes

1. Sample links back on main (80f9a315). The docs page pinned both sample links to the Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills modelcontextprotocol/csharp-sdk#1864 head commit. That works before merge, but afterwards it points readers at a frozen PR commit. Every other concepts page links samples under tree/main.

2. The frontmatter-verification caveat, said once per place it applies (0d936156). The point is correct and the spec requires it of hosts, but 6b0870cb repeats it across about ten API remarks, the docs page and both sample READMEs, sometimes several times on one type. I kept it where a reader acts on it:

   • SkillVerifier: the host's obligation, how to meet it with SkillFrontmatter.Parse + JsonNode.DeepEquals, and the full-parser alternative. The client extensions class, SkillFrontmatter and ReadSkillResourceAsync point there.
   • Skill.Frontmatter and IMcpSkillCatalog: the entry must match the file, and the SDK checks structure only.
   • McpServerSkill.Create(uri, JsonObject, files): what is still checked, and what the caller owns when the reader stops at unsupported YAML. The other explicit overload refers to it.
   • Docs page: a shorter responsibility table (server via McpServerSkill / server via custom catalog / host). Your host verification example in "Consuming skills" stays as is.

   The meaning of every statement is unchanged; this only removes repetition (86 insertions, 146 deletions). Your sample Program.cs change, which verifies frontmatter before displaying the file, is kept.

No code changes. The Skills package builds clean with warnings as errors on all target frameworks, and all 470 Skills tests pass on net10.0.

One more thing for modelcontextprotocol#1856

The modelcontextprotocol#1856 description still describes the original PR (for example, it says client wrappers are not included), so a reviewer arriving there gets the wrong picture. The real summary is only in your comment. Feel free to reuse whatever is useful from the modelcontextprotocol#1864 description. The "Relationship to modelcontextprotocol#1856" and "Authorship" sections would need inverting, of course.

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