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

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.

Start in install.sh and trace the successful-install path, including the existing ~/.local/bin PATH warning. Review the linked docs PR and sibling feedback issues for the read-the-numbers and feedback links. Done means a short, ordered, copy-pasteable block appears after a successful install without disrupting the warning.