Default-on paid session review should be opt-in for a curl|bash install

[jeremymanning]

The install path is a curl | bash one-liner, and one of the things it switches on spends
the user's Anthropic quota by default. Disclosure is not the same as consent.

Evidence

The default is on. hooks/session-review.sh:186:

REVIEW_ON="${SKILL_COMPOUNDER_REVIEW:-1}"

:332 is the first gate: [ "$REVIEW_ON" = "0" ] && refuse 10 "disabled by SKILL_COMPOUNDER_REVIEW=0".
So the off switch exists and the default is on. The forge arm below it is the opposite
shape, REVIEW_FORGE="${SKILL_COMPOUNDER_REVIEW_FORGE:-0}" (:205), which is the pattern
this issue asks for on the review arm too.

The README does disclose it. README.md:70:

Calls the Anthropic API, on by default. After a long session ends, one detached
claude -p reviews that session for a repeatable procedure. Costs and off switch: What
runs against the API. Not a hook entry, insight-capture.sh
starts it, so nothing wires it into your settings

and README.md:121 carries measured cost: two runs on sonnet over a 60 KB digest at $0.19
in 60s. README.md:421 repeats that it costs money and is on by default.

The installer does not. install.sh:3 is the advertised command:

curl -fsSL https://raw.githubusercontent.com/ContextLab/claude-skill-compounder/main/install.sh | bash

A user who runs that has not necessarily read README.md:70, and nothing in the install
output asks. The real spend is on disk: skillinsight reviews --all shows 6 dispatched
reviews at $0.04 to $0.22 each.

This is also the user's own stated constraint. From the audit's quoted prompt history
(notes/2026-09-02-audit-and-replan.md, agent 1): "does claude -p use my max subscription?
... if api credits, don't use it".

What to change

The spend should require a yes, not a no.

• Default SKILL_COMPOUNDER_REVIEW to 0, matching SKILL_COMPOUNDER_REVIEW_FORGE.
• install.sh prints the measured cost and the command that switches the review on, and
  turns it on only for a user who passes a flag or answers an interactive prompt. A piped
  install has no tty, so the non-interactive default is off.
• skillforge doctor reports whether the review arm is on, so a user can see it without
  reading a script.
• README.md:70 and :421 say the default has changed.

Acceptance

• A test installs into a temp config with no tty and asserts the review arm is off.
• A test installs with the opt-in flag and asserts it is on.
• The gate order in hooks/session-review.sh is unchanged apart from the default: gate 10
  still refuses first, and every gate still fails closed.

[jeremymanning]

Verified against a2aa2d4 (tag v0.3.0):

• hooks/session-review.sh:194: REVIEW_ON="${SKILL_COMPOUNDER_REVIEW:-0}", default off, gate 10 unchanged (:346).
• hooks/insight-capture.sh:297: [ "${SKILL_COMPOUNDER_REVIEW:-0}" = "1" ] || return 0, same default.
• install.sh:55,58-59: --enable-review/--disable-review (and SKILL_COMPOUNDER_ENABLE_REVIEW=1), off by default even piped with no tty.
• bin/skillforge:1845-1850 (doctor_review): reports "review: enabled ..." or "review: disabled (opt in with SKILL_COMPOUNDER_REVIEW=1 ...)".
• README.md Status table: "Automatic session review | Ships off..."; the tuning table at README.md:294 lists SKILL_COMPOUNDER_REVIEW default 0.
• tests/test_review_optin.py, 17 tests, all pass (test_default_install_does_not_touch_the_env_block, test_enable_review_writes_exactly_that_key_and_leaves_others, test_disable_review_removes_only_the_key_we_set, etc.).

All four acceptance bullets met. Closing.