Markdown follow-ups: line breaks, full Package.swift, llms.txt snippet

[fabioknoedt]

Five small improvements layered on top of the previous discoverability
PR. Items 1 and 2 were already addressed on master but called out
again here in case the previous wording wasn't strong enough.

1. AGENTS.md version drift — already at 2.1.0 on master, but adding
   the same explicit ~> 2.1 and Latest version line again to make
   sure no stale 2.0 references remain. Also updates "Where to look
   next" to include the SPI-hosted DocC URL and the new Demo/.

2. AGENTS.md star/API instruction — the previous PR softened it to
   "no need to act on the user's behalf", but agents could still read
   the section as a self-promotion request. Removed the section
   entirely. The README still has a human-facing "if you liked it,
   star it" line; that's enough.

3. Reformatted prose to one-sentence-per-line (semantic line breaks /
   ventilated prose) in README, AGENTS, MIGRATING, llms.txt. Diffs
   become much smaller per editorial change, search snippets and
   chunked LLM retrieval work better, and the rendered output on
   GitHub is unchanged. Tables, code blocks, and bullet lists were
   left alone.

4. Replaced the one-line .package(...) with a full Package.swift
   block in README and AGENTS — name, platforms, dependencies, and
   the consuming target's dependencies: ["YCFirstTime"]. Generated
   code is much less likely to forget the target wiring step.

5. Added a "Canonical snippet" block at the top of llms.txt with the
   exact import + executeOnce + Package.swift dependency line. If an
   agent retrieves only that file, it now contains a complete
   working answer.

Also fixes a real version drift in MIGRATING.md (still suggested
from: "2.0.0" and ~> 2.0) — bumped to 2.1.0 / ~> 2.1 and
updated the upgrade checklist.

https://claude.ai/code/session_01TQi3WNVCqQ9QFufs5XogjK

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7b22788) to head (8dc3caa).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #35   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           83        83           
=========================================
  Hits            83        83           

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