feat(reactor-cli): add docs-synthesis example (merged PRs → docs PR)

[irl-dan]

What

A new cost-tiered Reactor example at packages/reactor-cli/examples/docs-synthesis/, parallel to agent-observatory. It watches a code repo's merged PRs and opens one PR against the public docs site (openprose/docs) to keep it current — the worked example of the Cost and Context Discipline authoring guidance (#126), i.e. the PR→docs shape that guidance was written for.

The pipeline

repo-changes (gateway)     ~free: fold merged-PR deltas (paths + diffstat, no diff)
   └─ change-signal  (cheap classifier: reads ONLY the inline changed-path list)
        ├─ #cli-signal ───────► cli-docs
        ├─ #sdk-signal ───────► sdk-docs
        ├─ #reactor-signal ───► reactor-docs
        ├─ #devtools-signal ──► devtools-docs
        └─ #openprose-signal ─► openprose-docs   each: pending edits (page + why + cited PR)
                                   └─ docs-pr  (one coalesced PR against openprose/docs)

Each of the five public-docs sections subscribes to its own signal facet, so a PR touching only packages/reactor-cli/** moves #cli-signal only — cli-docs renders, the other four memo-skip at zero cost. A test/CI-only PR emits no signal at all. Cost scales with surprise, not with the clock.

How it maps to the discipline

• Tier; cheap gatekeeper filters surprise — change-signal is a narrow classifier (paths → signals), not a summarizer.
• Bound each narrow render to its inline input — classifier + accumulators read only the staged PR evidence; ### Invariants forbid running git / reading the repo. They emit pointers, not diffs. Each accumulator carries its section's fixed page list in-contract.
• Concentrate effectful work in the rare terminal node — docs-pr is the one render whose job is to act (the Tenet-2 caveat): it may read the specific pages it edits and the cited PRs, still bounded, and fires only when a pending-edit facet moved.
• Validate the cost-shape — committed replay/ + eval-harness (deterministic selective-wake check + opt-in judge panel).

Section → source mapping

signal facet	docs section	source it tracks
#cli-signal	cli/	packages/reactor-cli/**
#sdk-signal	sdk/	packages/reactor/** public API surface
#reactor-signal	reactor/	packages/reactor/** harness behavior + concepts
#devtools-signal	reactor-devtools/	packages/reactor-devtools/**
#openprose-signal	openprose/	skills/open-prose/**, references/*.md

Input

• Offline (default): four static PR fixtures in reactor.yml chosen to exercise selective wake — CLI-only (docs(examples): add top-level forwarding README so /tree/main/examples resolves #101), CLI+SDK (feat(evals): reactor harness evaluation suite (phase 1b) #102), openprose (docs: v0.1 launch surface — article, technical report, tagline qualifier #103), and a test-only no-op (feat(packaging): v0.1 npm headline demo — exports + example [parked: needs RC-verification update] #104).
• Live (opt-in): connectors.cjs.example reads git log --first-parent on the watched repo (no GitHub token), keyed github.

Status / follow-up

• reactor compile --check passes keyless; all contracts and the connector fixtures parse.
• Mirrors agent-observatory, which is not registered in the examples.test.ts fake-provider gate (that single-facet seam doesn't fit a multi-node pipeline) — validation is the committed replay/ + eval-harness.
• The committed replay/ requires a one-time keyed run (compile + serve with DOCS_SYNTH_DRY_RUN=1 so the actuator stays dry). Not included here — a guarded follow-up.

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