fix: allow rootless install and demo --force-procfs (#648)

[jenish-25]

What & why

Closes part of #648. The daemon already supports rootless operation via tracer init --force-procfs, but the installer and tracer demo never caught up — a non-root user (laptop, codespace, rootless container) is hard-gated at the env-check before the binary ever lands on disk, even though the runtime would support them.

⚠️ Draft / coordination: @WatchTree-19 filed #648 and offered to take all four parts. I picked it up as a first contribution — happy to hand off, split, or combine if you're already on it. cc @VaibhavUpreti @tiagonix

Changes

Installer (tracer-installer)

• Root check is now advisory instead of fatal, via a new InstallCheck::is_required() (defaults to true; RootCheck overrides to false). Non-root users see a WARNING and the install continues.
• Non-root installs now target ~/.local/bin/tracer instead of /usr/local/bin — the same location the updater already probes (update/updater.rs). Root installs are unchanged (/usr/local/bin).
• Post-install "Next steps" now prints the real install path and a rootless tracer init --force-procfs hint, replacing the stale ~/.tracerbio/bin/tracer chown/chmod block (that was a third, nonexistent path).

CLI (tracer)

• tracer demo gains --force-procfs, propagated into the init args. The flag was already documented in demo --help-advanced but not wired to a struct field, so it errored with unexpected argument. Now it works.

Docs

• README documents the rootless (--force-procfs) path for both init and demo.

On part 2 of the issue (--interactive-prompts none hang)

Already fixed on main — the auth path exits non-zero without prompting. Verified with the issue's exact repro:

$ tracer init --force-procfs --interactive-prompts none --pipeline-name x
Unable to log in automatically. Please open https://.../ and copy your init code here.
$ echo $?
1

So no change was needed there.

Testing

• New unit tests (TDD): install-dir selection (root vs. ~/.local/bin vs. no-home error), RootCheck is advisory, post-install setup_command, and demo --force-procfs propagation.
• Runtime-verified on macOS: tracer demo --force-procfs --help-advanced now parses (exit 0); an unknown flag still errors; the init repro above exits 1 fast.
• cargo fmt -- --check, cargo clippy --all-targets -- -D warnings, and the changed-crate test suites all pass on the pinned toolchain (1.91.1).

Open question for maintainers

tracer info shows tracer login && tracer init while the README leads with sudo tracer init --token …. #648 flags this as a fourth inconsistency. I left tracer info untouched to avoid changing product copy unilaterally — happy to align it either way if you have a preferred happy path.