Retire the P3/P4 spike branches and worktrees after #49 merges

[adbarc92]

Three spike branches remain, kept deliberately until PR #49's smoke passes — they are the only working reproduction if it fails.

Branch	Worktree	Notes
spike/app-plugins-webview	.claude/worktrees/spike+app-plugins-webview	P3 v1, superseded by v2 (2026-06-09)
spike/app-plugins-webview-v2	.claude/worktrees/agent-a709aaf1bcad07d41	P3 v2. Has uncommitted changes: a throwaway App.svelte harness marked "THROWAWAY — do not merge to main", plus untracked fleet.db* (gitignored on main, not on this June branch)
spike/view-plugins-handshake	—	P4, full GO recorded

Their load-bearing findings are already carried into embedding.rs / view_plugins.rs on #49, and the P3 results doc was rescued onto that branch in #49. The handoff explicitly designated the spike harnesses themselves as throwaway.

Do after #49 merges

git worktree remove .claude/worktrees/spike+app-plugins-webview
git worktree remove --force .claude/worktrees/agent-a709aaf1bcad07d41   # discards the throwaway harness
git branch -D spike/app-plugins-webview spike/app-plugins-webview-v2 spike/view-plugins-handshake

Check size first — the removed agent worktrees held ~1.8 GB of nested target/, and a large delete can outrun a short command timeout.

Do not do this before the smoke passes.