[repo]: split docs/ai into one file per entry

[seunlanlege]

The problem

Every package under sdk/packages/* kept three shared files in docs/ai/ — ChangeLog.md, Decisions.md and Flow.md. All three were append-only with newest entries on top, so any two concurrent PRs wrote the same line and GitHub marked both conflicted.

#1251 declared merge=union for the two log files. That fixes it locally and does nothing on GitHub, which ignores merge drivers when it computes a PR's mergeability.

Measured on #1250, which changes nothing but those two logs and one test file, and whose merge base is the .gitattributes commit itself:

Merge of #1250 into main	Result
locally, with merge=union	clean
locally, with the union lines removed	conflicts in ChangeLog.md and Decisions.md
GitHub	mergeable: false, dirty

Same two commits, same base SHA.

The change

Each entry is now its own file:

sdk/packages/simplex/docs/ai/
  README.md
  changelog/2026-09-10-guard-the-socket-mode-wizard-announcement-for-json-output-1237.md
  decisions/2026-09-10-run-s-flags-live-in-src-cli-run-options-ts-not-in-the-bin.md
  flows/order-intake-what-reaches-the-filler-at-all.md

Two PRs add two different files, so there is nothing to merge. It is the same reason changesets keeps one file per change instead of one shared changelog.

396 entries moved across core, indexer, sdk and simplex. The prose is untouched. The only edit is each entry's headings moving up one level, from ## to #, since an entry is now a document rather than a section.

.gitattributes is deleted — it had nothing left to declare. The root CLAUDE.md documents the layout and says not to gather entries back into a shared file, with the GitHub reason attached so nobody re-litigates it. Each package gained a docs/ai/README.md carrying the conventions that used to sit in the three preambles.

Flows stay one file per flow and are still edited in place, so two PRs revising the same flow still conflict. That one is worth seeing: it means two changes disagree about how the code runs.

Verification

The splitter is fence-aware. That mattered — the ## YYYY-MM-DD line inside each preamble's format example would otherwise have become a bogus entry.

Every entry was then reconstructed from the files on disk: demote the headings back a level, compare to the source text in the parent commit, byte for byte. All 396 recovered, and the per-directory file counts match exactly, so no two entries slugged to the same name and quietly overwrote each other.

Two branches off this commit, each adding a changelog entry, merge with no conflict.

For open PRs

#1250, #1246, #1234 and #1185 still append to the old files, so each hits a modify/delete conflict once. Resolving it means moving the entry into a new file under changelog/ or decisions/.