fix(#1421): resolve replace-image-src against md/mdx frontmatter

[davidwkeith]

Summary

• `apply_edit`'s `replace-image-src` op always refused with `no-match` / "no .mdoc files found" against the stock template's example photo page, because none of the existing resolvers (`.mdoc`, Keystatic, `.astro`) can patch an image whose `src` comes from Markdown/MDX frontmatter rendered through a dynamic expression (e.g. `` in `Hentry.astro`).
• Adds `resolveFrontmatterImage` to `server/patcher.mjs`'s resolver chain: for `replace-image-src` edits, it scans `.md`/`.mdx` files under `src/content` for a top-level frontmatter field (quoted or plain-scalar) whose value matches the drop target's current `src`, and patches just that value in place. Matches on value, not field name, since different content collections use different keys.
• No MCP schema change — reuses the existing `replace-image-src` op and payload shape.

Paired PR check

• This change is self-contained to the plugin (skills / hooks / template / docs / MCP server with no consumer-visible contract change).
• This change needs a paired PR in Anglesite/Anglesite-app.

Test plan

• Plugin self-checks pass (npm test — 3158 passed, 1 pre-existing unrelated failure in test/optimize-images-packaging.test.js due to a missing tsx binary in this fresh worktree, not touched by this change)
• Added test/fixtures/patcher/src/content/photos/*.md fixtures and describe("frontmatter image resolver", …) in test/patcher.test.js covering: quoted value match, unquoted plain-scalar match, no-match refusal, and ambiguous refusal (two files sharing the same image value)
• If touching the MCP server: paired app PR exercises the new/changed messages — N/A, no schema/op change

Closes Anglesite/Anglesite#1421