Agent sessions hand the operator URLs and they drown in terminal
scrollback. The Booth is the right home for them — it already has the one
property that decides adoption, which is that a session can publish with
mkdir and cp, no API key, no schema, no deploy — but everything in it dies
in 24h.
So: a booth containing `.forever` is never swept, and renders in its own
Kept lane at the top of the index. Opt-in per booth, so the ephemeral
default is untouched and nobody inherits a cleanup chore. `rm` the
sentinel and the board rejoins the sweep; the CLI verbs are sugar over
exactly that, which keeps the filesystem-is-the-state model honest.
The pin is deliberately NOT wired into is_expired(). That stays a pure age
question feeding the `expires_in` countdown; only sweep_once() honours the
sentinel. Keeping expiry arithmetic and reaper policy apart means they
cannot drift into each other.
Kept cards are visually separated per Australis: a 2px top edge in aurora
blue, the one accent border the system sanctions. They show "kept" instead
of a countdown, and they deliberately lose the one-click wipe button — a ×
next to the durable stuff is a footgun, so removing a kept board is a
two-step act.
`booth link <url> [description]` appends to the standing `links` board,
creating and keeping it on first use. Entries carry provenance (handle or
hostname, plus a timestamp) because a bare URL is unreadable three days
later. The append is one printf of one line to an O_APPEND fd — atomic
under PIPE_BUF on POSIX — which matters because many agents post to one
board and interleaved half-lines would be the obvious failure mode.
Seven tests cover the sentinel: detection, survival of a sweep that wipes
its neighbour, the deliberate is_expired/sweep_once split, the listing
flag, the sentinel not inflating item counts, and both lane-rendering
directions. Two of them originally asserted on the bare strings "Kept" and
"kept-grid", which passed for the wrong reason — those also appear in the
inlined stylesheet served on every page — so they now assert the full
class attribute. 55 pass.
Also corrects the Homepage card's description, which advertised a flat 24h
TTL that is no longer the whole story.
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