one-step maintainer activation / instant-value onboarding

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Goal: make the maintainer payoff (free anti-slop gate + AI second-opinion) self-evident at install.

Scope: on install / first dashboard visit, run the gate + AI review against the repo's recent PRs as a LIVE demo ("here's what gittensory would have flagged"), with a one-click advisory→blocking ramp. Reuse settings-preview + onboarding-pack. Coordinate with #647 / PR #648 (onboarding) — extend, don't duplicate.

Acceptance: a newly-installed maintainer sees a concrete repo-specific gate/AI preview in the first session and enables advisory mode in one action.

Depends on: #647 (onboarding). Effort: M. Sub-issue of #528.

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Inherits #525 cross-cutting AC: test:ci + 97% coverage; miner⊕maintainer access boundary (identity per-login, authority per-repo) + public-safe redaction (#542); advisory-first, no auto-merge/submit; no source upload from miner tooling. Part of #525.

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Non-visual activation service shipped in #708 (merged 8271bcf8).

Landed:

• buildMaintainerActivationPreview (src/services/maintainer-activation.ts): runs the deterministic advisory engine over the repo's recent PRs → "Gittensory reviewed your N most recent PRs and would have surfaced guidance on M of them" (per-PR findings + finding-code counts + current gate mode + AI-configured flag + recommended next action). Pure; never runs AI (no surprise cost) — only reports whether AI review is already on.
• One-click ramp: POST /v1/repos/:owner/:repo/activation enables the gate + deterministic rules in advisory (non-blocking) mode (advisory-first per the roadmap: contribution quality & adoption wave #525 AC), merged onto current settings. GET /v1/repos/:owner/:repo/activation-preview serves the demo. Both maintainer-scoped (requireRepoMaintainer).
• Reuses the gate engine + repo-settings infra (feat(github): authoritative .gittensory.yml gate config (config-as-code) #647/feat(ui): maintainer onboarding — hero install button + config-as-code docs #648); does NOT duplicate settings-preview (one hypothetical PR) vs. this (the repo's real recent PRs).

Remaining (owner-led / visual): the dashboard rendering of the activation preview on install / first visit (pairs with the dashboard work), and optionally a live AI preview over recent PRs (needs cached diffs + would incur cost — deliberately deferred).

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Discovery sweep (2026-06-17): ~60% complete. Advisory activation works; missing: owner one-click 'apply config' / remediation actions (today owners must copy-paste YAML into GitHub manually), inline focus-manifest fixes. Milestone M1.