docs: add Grant Radar example

[rawwerks]

Summary

• add the Grant Radar OpenProse example under skills/open-prose/examples/grant-radar/
• update the root and examples README indexes to point at the local example
• keep sync/provenance mechanics out of the user-facing example tree; upkeep lives in the upstream openprose/grant-finder contributor workflow

Validation

• npm --prefix tools/cli test
• npm --prefix tools/cli run typecheck
• upstream openprose/grant-finder: make validate && make validate-product-cli && make validate-recall && make dogfood-agent && make secret-scan
• no private validation artifacts, prompts, transcripts, or organization-specific holdout data committed

[irl-dan]

I worry the MIRROR will be confusing to someone popping into the examples unless we make this into some sort of first class pattern.

Options:

• we simply leave this example out of here for now, no harm in using it through the other repo
• we keep a single README document that points to the other repo, rather than mirroring it over here
• we duplicate it and don't try to link it in any way (ie thereby making this "the canonical one")

I'm mostly going for: keep "examples" as standard/vanilla as possible and don't introduce complications into the examples.

[rawwerks]

Agreed. I removed MIRROR.md and stripped the mirror/sync language from the user-facing example and example indexes. The PR now presents Grant Radar as a normal local example under skills/open-prose/examples/grant-radar/.

The sync mechanics are now kept out of the example tree: openprose/grant-finder has contributor-facing instructions plus an automated workflow that refreshes examples-mirror behind the scenes when examples/openprose/** changes. The source repo also has a portability validator so the example stays usable from either location without exposing the sync process to users.

Latest checks are green.