Split README into docs/, add a recorded demo GIF

[jsirish]

What

README.md is 492 lines with zero images. It's comprehensive but front-loads reference material
(full Configuration section, git-worktree resolution details, housekeeping/retention policy, the
repo-layout tree) ahead of anything that shows the thing working. There's a promo site
(docs/index.html, https://dynamic.github.io/throughline/) that already carries the narrative
pitch, so the README's job is narrower than what it's currently doing.

Prior art

adrrr/persistent-handoff's README is 115 lines (down from 204 in an earlier revision,
per their CHANGELOG — deliberately restructured "around its first screen"): pitch → animated
demo GIF with a one-line caption → an anchor row (Example · Install · Try it · Contract · FAQ · Related) → install. Reference material (derived-path digest mechanics, ${CLAUDE_PROJECT_DIR}
expansion gotchas, the character cap, resume deduplication) moved to docs/REFERENCE.md;
per-harness install/uninstall/hand-install detail moved to docs/INSTALL.md.

Their demo GIF is checked in with its recording script (demo.tape, using VHS or similar), and
the tape explicitly waits on a "skill fired" marker line so a take where the skill silently
doesn't trigger times out during recording instead of shipping a misleading GIF.

Fix shape

1. Move ## Configuration (worktrees, opt-out, machine-wide disable), ## Housekeeping, and
   ## Layout into a new docs/REFERENCE.md.
2. Move per-harness install detail (already partially duplicated with the promo site) into
   docs/INSTALL.md, keeping only the one-liner install commands in the README itself.
3. Record a short terminal GIF showing: a session running normally (hooks capturing silently), a
   /handoff producing a HANDOFF.md update, then a fresh session's SessionStart picking it up.
   Check in whatever recording script produces it (VHS .tape is the tool persistent-handoff
   uses) so the GIF is reproducible, not a one-off screen capture.
4. Restructure the README's opening to pitch → GIF → anchor row → install, matching the "first
   screen" pattern above.

Depends loosely on # for having something concrete to record — filing that
issue alongside this one.