Follow-up to #57466 simplifying `load_skill_frontmatter` rather than
fixing a separate bug.

#57466 fixed the chunked-read loop so it truncates accumulated bytes at
the frontmatter boundary, preventing `str::from_utf8` from failing on a
multi-byte grapheme split across chunks. That fix is correct, but the
chunked read it's working around isn't pulling its weight:
`MAX_SKILL_FILE_SIZE` is 100KB and there's already a metadata pre-check,
so "stop reading early once we've seen the closing `---`" saves at most
~25 pages per file while forcing the use of `open_sync`, a hand-rolled
loop, and a long comment about `smol::unblock` vs the GPUI test
scheduler.

This PR:

- replaces the chunked `open_sync` + read loop with a single
`fs.load(...)` call (the same primitive `read_skill_body` already uses);
- deletes `closing_delimiter_end`, `SKILL_READ_CHUNK_SIZE`, the
`std::io::{self, Read}` import, and the `Parking forbidden` paragraph;
- tightens the metadata pre-check so a metadata error (permissions, I/O,
etc.) bails out instead of silently falling through to a blind read;
- removes the now-obsolete
`test_load_skill_frontmatter_with_emoji_at_chunk_boundary` test — by
construction, a single full read can't split a UTF-8 sequence at any
boundary.

Closes AI-303

Release Notes:

- N/A