feat(lsp): add Marksman language server for Markdown

[jpds]

Issue for this PR

None.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds support for using https://github.com/artempyanykh/marksman as an LSP for Markdown.

How did you verify your code works?

Deployed the Nix flake for this branch and verified by having an LLM create and navigate a fake notes collection.

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[Enough1122]

AI code review — automated review for reference; please use your judgment.

1. packages/opencode/src/lsp/language.ts — unlike the sibling Taplo change (feat(lsp): add Taplo language server for TOML #42937), this PR adds the server but does not register .md/.markdown in LANGUAGE_EXTENSIONS; if that map participates in deciding which built-in server attaches to a file (as .toml": "toml" just did), Marksman may never be selected despite being registered — please either add the two mappings for consistency or explain why Markdown is exempt.

2. packages/opencode/src/lsp/server.ts:1988 — spawn(bin, ["server"]) relies on stdio being Marksman's default transport (it is today); passing --stdio explicitly would pin the contract against a future CLI default change — cheap insurance for a long-lived builtin entry.

3. Nit — packages/web/src/content/docs/lsp.mdx:33 — same follow-up as other manual servers: Marksman ships single static binaries on GitHub releases, so an auto-install path would spare most users the manual step; fine to defer.

Overall: clean addition following the established builtin-server pattern (registration, graceful missing-binary no-op, root markers, alphabetized docs). Item 1 is the one thing to verify before merge. Thanks!

[jpds]

Point 1 isn't relevant - Markdown is already in LANGUAGE_EXTENSIONS.

As a Nix user, 3 and the concept of auto-installing things from the internet gets a "no" from me.

[github-actions]

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