docs: state what is actually built, and what is not - #68

Both tracked design docs said "Status: proposed" for work that shipped months
ago. The memory doc in particular was edited across #64/#65/#66 while its own
header claimed none of it existed.

Replaced the headers and the milestone list with a state AUDITED against the
code rather than recalled:

  sessions-experience    E1/E3 shipped, E4 partial, E2 NOT STARTED
  sessions-memory        M1-M4 complete

The finding worth having: **E2 — the switcher and in-panel search — is entirely
unbuilt.** No search input, no filter chips, no scope switch, no keybindings;
`AI: Sessions` is the only command. That is the largest remaining gap in the
experience and the one users hit first once a project holds more than a
screenful of sessions: the panel scrolls but does not search.

Also corrected: the FLIP filter reflow was listed in E4 as an independent item,
but it cannot ship before E2 — it would have no filters to reflow.

Scope note: only TWO of these docs are in the repo. levelcode-chat-sessions-
design.md and MEMORIES.md are UNTRACKED local drafts, so they are not repo
documentation and not mine to commit. `git grep` had been silently skipping
them, which is how they were miscounted as part of this debt in the first place.

No code changed.

Review found the Status lines linking to ./levelcode-chat-sessions-design.md,
which 404s on GitHub. The link was not wrong; the file was held back by a line in
.git/info/exclude — a LOCAL, per-clone exclusion that is never committed, so the
doc existed on one machine and nowhere else.

Publishing it rather than de-linking, because four tracked files send readers
there and two of them are SOURCE:

  docs/levelcode-sessions-experience.md    links to it
  docs/levelcode-sessions-memory.md        links to it
  extensions/levelcode-ai/sessionStore.js  "The design lives in
  extensions/levelcode-ai/sessionEvents.js   docs/levelcode-chat-sessions-design.md"

Anyone cloning this repo reads those two headers and finds nothing. 279 lines,
zero TODO/FIXME markers — finished prose describing shipped work.

It carries the same audited status as its siblings, from the same pass over the
code: Phases 1-3 shipped; Phase 4 partial — pins, 30-day auto-archive, Undo
behind Done/Delete, the sparkline, files chips, state pills, empty states and
Copy-as-Markdown are in; in-panel search and the switcher are NOT. Phase 4's
"polish" was hiding that, which is the same omission E2 was hiding in the
experience doc.

docs/MEMORIES.md stays untracked, deliberately and unchanged. Its only reference
is in an UNCOMMITTED CLAUDE.md edit, so it breaks nothing today, and it describes
a memories.js that was never built — committing it would reintroduce the exact
"docs claim work that does not exist" problem this branch exists to remove.