feature: install.sh — print next steps and a feedback link on successful install

[mrsabath]

Feature Description

When install.sh finishes, the single biggest lever on whether a user reaches "first useful reading"
is telling them the exact next command. A user who installs and then stops because they do not know
what to run next has churned before they saw any value.

Proposed Solution

• On a successful install, print the next steps in order: run abctl observe in one terminal, run
  your agent in another, then read the numbers (link to the read-the-numbers page).
• Include the feedback link in the same block (see the sibling docs and abctl issues).
• Keep the block short and copy-pasteable. If ~/.local/bin is not on PATH, the message already
  warns about that; the next-steps block comes after it.

Additional Context

Follow-up to the docs PR rossoctl/rossoctl#2551,
sibling to the docs feedback-template issue and the abctl footer issue. This is the tool-side half
of the first-run path (#959): the docs describe the
spine, and the installer prints the first step of it.

Supports exit criterion 5 of the Cortex v0.9.0 epic
as a follow-up, not release-blocking. Depends on the v0.9.0 install.sh landing (see
#944 / #945).

Owner area: install / service lifecycle.

[mrsabath]

Holding this one until install.sh exists.

I picked this up alongside its siblings (#975 → PR #978, #977 → PR #979 + a docs commit), but this issue is about the success message printed by install.sh, and that script is not in main yet — the repo currently ships only authbridge/install-demo.sh. The v0.9.0 install.sh is the target of the verified-install work (#944 / #945), which this issue's own dependency note already calls out.

Adding a next-steps + feedback block to install-demo.sh now would document a script the release is replacing, so the cleaner path is to wait and land this together with install.sh when it arrives.

Ready to go the moment install.sh lands. The block should:

• Print the first step of the first-run path (docs: first-run path — install to first useful reading #959): run abctl observe in one terminal, run your agent in another, then read the numbers (link to the read-the-numbers page).
• Include the feedback link — pointing at the same place as the abctl footer (feature: abctl observe — show a feedback/next-steps link in the footer #975) and the docs prompt (docs: add a laptop feedback path — issue template and an end-of-page 'give feedback' call #977): the Laptop feedback issue form on rossoctl/cortex.
• Come after the existing PATH warning, and stay short and copy-pasteable.

No PR yet, by design. Assisted-By: Claude Code