Never let one unreadable line take a transcript down

Removing `Event::TaskNote` hours after adding it made every transcript that
had recorded one unreadable. `Transcript::open` parses every line, so `launch`
failed for those sessions and `SessionManager::new` logged
"couldn't relaunch session <id>" and skipped them -- and a skipped session has
no pump and no driver. On the phone that is no status, no history and nothing
sendable, for every live session that had run a background task. One
unfamiliar word took down every conversation it appeared in.

A transcript is append-only and permanent, so the set of kinds one can hold
only ever grows: what this build writes is not what it may have to read. A
line can come from a newer server, or from an older one that wrote a kind
since dropped, and neither may be able to end the file.

`Indexed::parse_at` degrades a line it cannot make sense of to
`Event::Unreadable { kind }` instead of failing the whole read. It keeps the
line's seq -- the cursors, the page bisection and the next-seq counter are all
addressed by it, and dropping the line would hand out a seq the file already
contains -- and carries the word the line called itself, so the phone can say
what is missing rather than that something is. A line with no readable seq is
still an error: that one cannot be placed at all.

`Event::TaskNote` comes back retired rather than deleted: deserializable,
never constructed, dated, with the reason on it. The phone folds it to no row,
which is the point -- an unreadable line correctly draws a placeholder, and
one per background task is the wall the row was removed for in the first
place.

Found while diagnosing a report that live sessions had lost their status and
could not be sent to. 173 server tests pass, including the new one, which
fails on the old code within a second.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -296,6 +296,19 @@ written, and the fold uses that same predicate to decide a reply is settled.
 
 ## Things that have bitten
 
+- **A transcript outlives the enum.** Removing `Event::TaskNote` hours after
+  adding it made every transcript that had recorded one unreadable, so
+  `launch` failed for those sessions and `SessionManager::new` skipped them —
+  no status, nothing sendable, no new messages, for every live session that
+  had run a background task. **The set of kinds a transcript can hold only
+  ever grows**: a line may come from a newer server or from an older one that
+  wrote a kind since dropped, and one unfamiliar word must never be able to
+  end the file. `Indexed::parse_at` degrades a line it cannot read to
+  `Event::Unreadable { kind }`, keeping its seq — which is what everything
+  downstream is addressed by — and the phone draws it as a placeholder saying
+  which kind. Never delete a variant instead of retiring it; `Event::TaskNote`
+  is what retiring looks like, and the phone folds it to no row.
+
 Project-specific only — a lesson that would bite any project on this machine
 belongs in `~/.claude/TOOLCHAIN.md` or `~/.claude/MACHINE.md` instead.