[nicosuave]

Adds Rex support alongside the herdr plugin.

Rex has no plugin manager. A plugin is a Lua module that init.lua loads with require (the rex-worktrunk pattern). memex ships one at rex/memex.lua, embedded in the binary, and memex rex install writes it beside Rex's init.lua. It never edits init.lua; it prints the lines to add.

require("memex").setup{}
rex.bind("cmd+shift+m", "memex_palette")

Actions (they mirror the herdr plugin; all appear in Rex's command palette under "memex"):

• memex_palette: the TUI zoomed over the focused pane. Quitting it restores the layout.
• memex_here: the palette filtered to the focused terminal's repo.
• memex_desk: a memex tab, reused when one is already open.
• memex_sidebar: a split beside the focused pane, or closes it if one is open.
• memex_resume_last, memex_index, memex_web.

Resume inside Rex. When memex runs in a Rex terminal (REX_SESSION is set), resuming from the TUI, or with the hidden memex rex resume[-last], works like this:

• It opens a new Rex tab at the session's directory and types the resume command into the tab's shell, via initial_input. The shell stays open after the agent exits, as with herdr's pane run.
• If a terminal in the same Rex session is already running that session, memex focuses it instead. It matches the session id against the command lines of the processes under each terminal.
• rex_resume = "tab" | "split" | "off" in memex's config.toml changes where sessions open.
• Session selection is now shared with memex herdr resume[-last] (resume_target).

Rex-specific choices:

• Palette placement. The palette is a zoomed split rather than a floating layer. Rex draws floating layers without a backdrop, and the TUI keeps the terminal's default background, so the pane underneath would show through.
• Pane labels. Resumed panes carry no pane label; only their tab is labeled. The module finds its own panes by the label memex, which would otherwise match an agent resumed from the memex repo.
• No startup hook. Rex doesn't send events to config files. The memex daemon already covers background indexing.

Checked:

• clippy with warnings as errors is clean.
• New unit tests in src/rex.rs and a memex rex install integration test (tests/rex_cli.rs) pass, along with cli_surface.
• The Lua module passes rex config check (7 actions).
• Live against Rex 0.0.1792787366 with a sandbox root: memex rex resume-last opened a tab running the resume command, and a second run focused that tab. memex_palette, loaded through init.lua, opened the TUI zoomed over the pane.
• Not yet exercised live: desk, sidebar, here, index, web.