Open exported HTML after confirmation

[forhappy]

Summary

• track generated HTML pages explicitly in successful CLI outcomes
• ask interactive Compass users before opening semantic diff, graph, call-flow, or tree HTML
• launch the default browser on macOS, Linux, and Windows only after an explicit y or yes
• skip prompts and browser launches for scripts, pipes, redirected commands, CI, failed exports, and --no-viz
• preserve Graphify compatibility behavior and document the Compass terminal policy

Why

Compass HTML commands currently write a page and require the user to locate and open it manually. This adds one shared, safe post-export workflow while keeping automation non-interactive and defaulting the confirmation to no.

Validation

• cargo check -p compass-cli
• cargo test -p compass-cli --lib — 61 passed
• cargo test -p compass-cli --test help_cli — 6 passed
• cargo fmt --all -- --check
• git diff --check
• real pseudo-terminal checks for compass tree and compass export html; each generated its page, displayed Open … in your browser? [y/N], and remained closed after n
• graphify update .

Notes

• The Python/Rust tree parity suite could not start because .venv/bin/python is absent in this worktree.
• Strict workspace Clippy remains blocked by existing unrelated warnings in compass-output and semantic_diff_render.