docs: reposition README + tighten CONCEPTS for the audience

[phall1]

What

Two tightening commits (already on the branch) plus a README reposition aimed at the actual audience: senior engineers, AI-slop-saturated, deciding in 60 seconds.

README rewrite

• Thesis-first hook. Leads with the knife — the same VT parser runs on both ends, so modern protocols survive a remote attach tmux would smear; and the addressable unit is the terminal, not the session — instead of the implementation ("a libghostty-backed control plane").
• Agent/headless surface above the fold. send-keys / run / wait / watch + phux-mcp ship and are tested. The old README buried them under three tiers of "roadmapped." They're the story for this audience.
• Honest status. Replaced roadmap-as-features (the docs(adr-0018): revise state-sync implementation gate — RenderState is the cache primitive #1 vaporware tell) with a blunt split: stable / shipped-but-API-moving / not-yet. Federation correctly scoped to "addressing in the wire, routing not yet."
• Crates table updated for phux-client-core (compiler-enforced ratatui boundary) and phux-mcp.
• Demo placeholder left as an HTML comment — a ~10s cast of kitty graphics surviving detach/reattach is the highest-leverage missing asset.

CONCEPTS tighten (pre-existing branch commits)

• Killed the Philosophy preamble, the Hashimoto quote, the "When to use phux" section; reframed "unit of work" as "Terminals, not panes"; tightened the comparison table.

Notes

• just docs-check green (75 files, 0 violations).
• Rebased clean on current main (was 29 behind; README/CONCEPTS untouched upstream).
• No demo asset committed yet — placeholder comment marks where it goes.

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