Add the phone mockups and the design decisions behind them

What this adds

docs/ui-prototypes/mobile/ — three phone-frame HTML mockups (390×844, dark and light; two frames are interactive) and a README that records the owner's reactions as a decisions list, so the screen issues can be refined from it.

• 01-home-and-session.html — sessions home and session page
• 02-composer-and-pickers.html — where the model/mode/effort picks live (four options; B is clickable), model sheet, mode/effort popovers, full-screen composer
• 03-environments-pairing-drafts.html — settings, environment detail, pairing (scan a QR or paste a link, interactive), drafts tab, project picker

Every colour is a web token name lifted from globals.css / fluid.css, so the frames double as the spec for the token pipeline.

docs/ui-prototypes/ was gitignored as a scratch folder for local experiments; it stays that way, with only the mobile/ folder un-ignored.

Decisions recorded

• Picks move out of the session header into the composer: picks row at rest, one summary chip while a turn runs. The header is title + project only.
• Glass on every floating surface, with a per-scheme --glass recipe.
• Pill nav is Sessions · Inbox · Settings, search detached, compose top-right.
• Home tabs All · Active · Drafts; one status glyph per state.
• Still open: Inbox and search screens, tool-row detail, other environments' drafts placement.

No code changes; docs only.

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