feat(home): browser terminal for the sandbox; herdr as its own aggregate; aspects charter

[cameronraysmith]

Summary

PR-A of a three-PR stack (A: sandbox terminal → B: dissolve packages → C: module adoption). Tracked in Linear as CAM-44.

The Devin sandbox's built-in editor hangs in the browser-to-VM tunnel while the VM-side server stays healthy, so the sandbox needs an interactive terminal that does not depend on it. This PR provides one declaratively and, in doing so, fixes two aggregate-boundary defects it exposed.

• modules/home/README.md (new): the home-manager aspects index and the aggregate contract that PR-B and PR-C are verified against — one directory = one user-facing capability, one secret posture; module over raw package; configure at the consumer; per-user exceptions are lib.mkForce overrides, not omitted aggregates; no Den/flake-file, no _*.nix escape modules.
• herdr split out of ai into modules/home/herdr/ (git mv, aggregate key ai → herdr): it reads no secret, so a user without the AI secrets can import it. crs58 imports both; its package set is unchanged except +browser-terminal.
• modules/home/herdr/browser-terminal.nix: writeShellApplication around ttyd -W … herdr, port 7681, bind 127.0.0.1 (override BROWSER_TERMINAL_INTERFACE/BROWSER_TERMINAL_PORT), COLORTERM=truecolor. It lives in herdr because it consumes both herdr and ttyd. The xterm.js fontFamily list is pinned in the wrapper because the browser resolves it against the client's fonts; Nerd Font glyphs are private-use codepoints and never come from fonts installed in this profile.
• modules/home/terminal/ttyd.nix: raw pkgs.ttyd, unconditional, replacing the line in packages/development-packages.nix so users importing terminal without herdr keep it (verified: christophersmith, tara package sets identical to main).
• users/ubuntu imports core + herdr and overrides what the sandbox cannot run: services.ssh-agent.enable = mkForce false (no systemd user manager) and LANG/LC_ALL/LC_CTYPE = mkForce "C.UTF-8" (the image's glibc has only C.utf8; core's en_US.UTF-8 made every shell warn).
• .devin/blueprint.yaml knowledge item terminal: a session starts browser-terminal and exposes 7681 through the authenticated preview tunnel; initialize/maintenance never do (no tunnel during image builds); the tunnel is the access control.

Verification: nix flake check, just lint, statix/deadnix, browser-terminal derivation built (shellcheck), ubuntu@x86_64-linux activationPackage built; sorted home.packages names vs main: crs58 +browser-terminal; ubuntu +browser-terminal +htab +herdr +ttyd +2 fontconfig dirs; christophersmith/tara no change. Activated on the Devin VM and confirmed through the preview tunnel: ttyd → herdr → fish, Nerd Font glyphs render, nvim works.

Link to Devin session: https://app.devin.ai/sessions/a940684e0e14430f82ed19713c2d0385
Open in Devin Desktop: https://app.devin.ai/desktop/session/a940684e0e14430f82ed19713c2d0385?variant=devin
Requested by: @cameronraysmith

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