docs(skills/acpx): bring SKILL.md up to 0.7 surface

[superWorldSavior]

What

Updates skills/acpx/SKILL.md so the bundled skill reflects the 0.7 surface instead of the pre-0.6 subset.

Why

The skill is the canonical reference an agent loads when it sees acpx mentioned. Today it is missing every flag and subcommand added in 0.6 and 0.7, so an agent asking "how do I prune old sessions?" or "how do I author a flow with decision()?" gets nothing useful and falls back to guessing. This PR makes the doc match what the binary already does.

What's added

Frontmatter description: now mentions system-prompt overrides and defineFlow/decision/decisionEdge so triggering reflects 0.7 use cases.

Core capabilities (bullet list): 5 new lines covering sessions ensure, sessions prune, system-prompt overrides, --no-terminal, --allowed-tools/--max-turns/--prompt-retries, and the flows API.

Command model: sessions ensure, sessions prune [--dry-run] [--before <date> | --older-than <days>] [--include-history], and flow run <file> [--input-json | --input-file] [--default-agent].

Sessions section: ensure (idempotent get-or-create) and prune examples + behavior notes (retention by close time vs last-used, --include-history event-stream cleanup).

Global options — adds:

• --non-interactive-permissions <deny|fail>
• --json-strict
• --system-prompt <text> / --append-system-prompt <text>
• --allowed-tools <list>
• --max-turns <count>
• --prompt-retries <count>
• --no-terminal

New section: System prompt override (Claude) — documents the _meta.systemPrompt / _meta.systemPrompt.append forwarding, persistence into session_options.system_prompt, and the cross-agent ignore semantics. Includes a claude -s reviewer example.

New section: Sessions cleanup — retention guidance and prune examples.

New section: Flows (multi-agent workflows) — replaces the two-line "Flow run" mention with:

• acpx flow run invocations (file + JSON input, --default-agent)
• A defineFlow example (pr-triage with decision, acp, checkpoint, decisionEdge) modeled after examples/flows/branch.flow.ts
• Node-type table: acp, decision, action, compute, checkpoint
• Three edge shapes: linear to, JSONPath switch, exhaustive decisionEdge
• Short rationale: cross-vendor by construction, persistence/replay, permission preflight, typed routing

Practical workflows — 4 new examples: persistent --system-prompt Claude reviewer, sessions ensure script bootstrap, sessions prune retention, multi-agent flow run.

Sources

Every claim and signature in this diff is derived from one of:

• docs/CLI.md — sessions subcommands, flow run, global options
• docs/flows.md — flows authoring API
• examples/flows/branch.flow.ts — decision({ choices, question }) and decisionEdge({ from, choices, cases }) shapes
• acpx 0.7.0 --help — every flag listed in this PR
• CHANGELOG.md — 0.6.0 and 0.7.0 entries

I did not invent any shape or flag. Where the source is silent (e.g., the full action/compute parameter sets), the doc stays minimal rather than guessing.

Testing

Doc-only change. No source code touched, so pnpm build && pnpm check && pnpm test:coverage should be unaffected. I have not run them locally — relying on CI.

I have not type-checked the embedded code samples in a scratch project. They are written to mirror examples/flows/branch.flow.ts shapes; happy to validate against a real acpx/flows import if maintainers prefer.

AI disclosure

Per CONTRIBUTING.md:

• AI-assisted: drafted with Claude Code (Opus 4.7). I cross-checked the claims against the sources listed above before opening the PR.
• Testing degree: untested beyond reading the diff. No new functional code; the change is documentation.
• Confirmed understanding: the authoring shapes ({ choices, question }, { from, choices, cases }) and CLI flag semantics (_meta.systemPrompt forwarding, prune retention behavior) are pulled from the source files in this repo, not from training data or third-party blog posts.
• I will resolve bot review conversations I address before requesting re-review.

Out of scope

• No example flows added or changed.
• No source code modified.
• action/compute/checkpoint parameters are documented only at the level present in docs/flows.md.

Happy to split, trim, or expand any section based on review.