docs: add agents.md, an entrypoint for coding agents

[bartlomieju]

Coding agents arrive at a Deno project carrying wrong priors: that Deno is a
separate ecosystem, that adopting it means rewriting imports or moving to JSR,
that package.json has to go. We already maintain agent skills that correct
all of this, but an agent only benefits from them once they are installed, and
nothing in the CLI or the docs currently tells it they exist.

So this page is deliberately an entrypoint rather than a guide. It orients the
agent, states the Node assumptions to unlearn, and then hands off to the skills
in denoland/skills, which stay the single source of truth for the command
surface, permissions, configuration layout, and migration. For agents that
cannot install skills it links the raw SKILL.md files, which are always
current, so nothing here needs restating and there is no second copy to drift.

The file lives in static/, so it is copied verbatim and served as
text/markdown by the existing markdown-source middleware. The URL we intend
to advertise is deno.com/agents.md; a companion dotcom PR serves this content
there, and a deno PR adds the link to deno --help.

[bartlomieju]

If agent installs Deno as the first step this should probably say dx skills ...

[bartlomieju]

Good catch — fixed. Switched to dx skills add denoland/skills --skill deno, since section 1 has just installed Deno and dx ships with the binary, so this no longer assumes Node is present. Verified dx skills resolves and runs the CLI. Left npx skills add ... mentioned as the identical Node alternative.