[3.0] Publish a batteries-included artifact so jbang users get the default extensions

[jbachorik]

Problem

The two primary install paths ship different capabilities:

Install path	What you get
jbang io.btrace:btrace (Maven Central)	engine only — no extensions
Distribution (SDKMAN / tar-zip / $BTRACE_HOME)	engine + bundled default extensions in extensions/
Custom fat agent (fatAgentJar -PembedExtensions=… / Maven fat-agent goal)	engine + chosen extensions, embedded

Root cause: the published io.btrace:btrace masked JAR depends only on btrace-agent/btrace-client/btrace-compiler (btrace-dist/build.gradle:115-117), so it contains zero extension implementations and sets no BTrace-Embedded-Extensions manifest entry. Under jbang, BTRACE_HOME resolves to the ~/.m2/.../btrace-<version>.jar location, which has no sibling extensions/ dir — so both discovery paths (filesystem scan of $BTRACE_HOME/extensions/ and embedded-from-resources) come up empty (Main.java:418,426). Users get core tracing (probes, BTraceUtils, aggregations, profiling, JFR) but none of metrics/statsd/contracts/llm-trace/etc.

Decision

Publish a second, batteries-included artifact that embeds the default extensions, while keeping the lean io.btrace:btrace for users who want just the engine.

• Lean: jbang io.btrace:btrace → engine only (unchanged).
• Batteries-included: jbang io.btrace:btrace-all → engine + default extensions embedded. (artifact name btrace-all is a suggestion — confirm before implementing.)

Work

• Add a btrace-all publication to btrace-dist that produces the fat agent with the default (allow-listed) extensions embedded — reuse the existing btraceFatAgent/fatAgentJar mechanism (build.gradle:459+) with autoDiscover limited to the default set.
• Publish + sign the new artifact in release.yml alongside io.btrace:btrace (currently only :btrace-dist:publishAllPublicationsToSonatypeRepository → the single btrace artifact).
• Update jbang docs/aliases: document both the lean and batteries-included one-liners and what each includes.
• Add a smoke test that a script using a default extension (e.g. metrics) links successfully when run from the batteries-included artifact and fails cleanly (with a clear message) from the lean one.

Depends on / relates to

• Depends on [3.0] Extensions pre-release fixes (dist bundles test/examples; statsd DNS on probe thread; stddev overflow) #889 — the extension-bundling allow-list. The batteries-included artifact must embed only the real default extensions, not the btrace-ext-test fixture or the Spark/Hadoop examples.
• Relates to [3.0] Published Maven surface omits coordinates the README and plugins resolve #884 — the publication-surface decision (only the masked jar is published for libraries); this adds one deliberate, additional published artifact on top of that.