First batch of #713 (agent per-container UI prose migration).
Moves 3 substantive WHY-explanations from app.js into
docs/web-ui.md::Per-agent page, replaces each in-code mention
with a brief pointer.
Moved to docs:
- **Terminal-wrap pill anchor** (~11 lines): why the `↓ N new`
pill is anchored in `.agent-main` rather than the default
`.terminal-wrap` parent — backdrop-filter creates a stacking
context, anchoring inside it traps the pill's z-index below the
composer.
- **OAuth code input** (~14 lines across two blocks): masked
password + reveal toggle + `autocomplete="one-time-code"`
rationale (shoulder-surfer / screenshot exposure + WHATWG
semantic value + suppresses browser save-password prompt).
- **NavLink kind resolution + XSS-safe DOM-build** (~8 lines):
Container → same-origin, Forge → `http://<host>:3000<url>`,
External → already absolute; agent-declared strings never reach
innerHTML.
Collapsed in app.js: each block becomes a 3-4 line pointer to the
matching docs section. Drops #262 / #568 / #592 cookies en passant
since their substance now lives in docs. The `#14` cookie on
`historyUrl` / `streamUrl` is also scrubbed — the path-relative
shape is the convention, not an open issue. `#375` (agent.css ID
ref) drops as part of the pill-anchor block collapse since it was
sole-purpose pointing inside that comment.
app.js: 27 → 24 refs. Net ~35 lines of substantive prose moved out
of app.js into docs/web-ui.md::Per-agent page, where it belongs.
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