feat(zcode-plugin-langfuse): add Langfuse observability plugin

[erlinerd]

Summary

Adds zcode-plugin-langfuse, a community ZCode Hook plugin that sends one
fail-open Langfuse trace named ZCode Turn per completed turn.

• Tool calls are recorded as spans and the assistant response as a generation.
• The plugin publishes only on Stop and reads only fields delivered by ZCode
  Hook stdin.
• It never reads transcript files or collects hidden chain-of-thought.
• Missing credentials, malformed input, local state errors, and Langfuse errors
  do not block ZCode.
• Prompt, tool-input, and tool-output capture are independently switchable and
  size-bounded.

Update: official plugin template layout (2026-09-14, v0.2.3)

This head reworks the shipped plugin directory to match the official template
(zcode-plugins-official/example-plugin) layout, generated by the source
repository's unified build (npm run package:plugin) at source tag
v0.2.3
(commit 134b4fe):

• plugins/zcode-plugin-langfuse/ now contains exactly 8 files:
  .zcode-plugin/plugin.json, .claude-plugin/plugin.json (identical copy),
  hooks/hooks.json (pointing at hooks/entry.mjs), the sealed
  hooks/entry.mjs esbuild bundle, README.md, README_CN.md, LICENSE,
  and THIRD_PARTY_NOTICES.md.
• The payload/ nesting and the artifact package.json are gone. Verified
  with diff -r against the source build output: byte-for-byte identical.
• Supersedes the 2026-09-13 update below: the entry point is now
  hooks/entry.mjs (official template path), not dist/hooks/entry.mjs;
  the artifacts/plugin-layout staging path no longer exists (the build now
  emits dist/plugins/<name>/ directly).

Marketplace registration (updated in this push)

• Plugin directory: plugins/zcode-plugin-langfuse
• Plugin version: 0.2.3
• Category: developer-tools (moved from the proposed observability, which
  is not in the validator's allowed set)
• The marketplace entry now carries description_i18n (en + zh-CN)
  deep-equal to .zcode-plugin/plugin.json, per the validator rule.
• python3 scripts/validate.py on this head reports
  OK: 20 plugin(s) validated (exit 0); git diff --check is clean.

Review fixes

• Process environment now overrides persisted plugin options, and persisted
  options are selected only for the exact ZCODE_PLUGIN_ID.
• Langfuse endpoints are HTTPS-only; plaintext HTTP is rejected.
• Every captured value, including truncation markers, stays within
  max_capture_chars.
• README files document six process hooks, current-user permissions, config
  reads, local state writes, HTTPS network access, command-execution boundaries,
  and transcript/privacy boundaries.
• THIRD_PARTY_NOTICES.md identifies langfuse 3.38.20,
  langfuse-core 3.38.20, and mustache 4.2.0, all MIT-licensed, with
  provenance links.

Provenance and side effects

• Source repository: https://github.com/erlinerd/zcode-plugin-langfuse
• The bundle inlines the official langfuse JavaScript SDK, so the catalog
  copy needs no runtime npm install; it talks to the same Langfuse HTTPS
  ingestion API through Node's built-in fetch.
• Network access: HTTPS requests to the configured Langfuse endpoint, only for
  telemetry publishing.
• Local writes: bounded per-session state under ZCODE_PLUGIN_DATA when
  available, or the ZCode plugin data fallback; state is cleaned up after a
  completed Stop.
• Each of the six hooks runs node with the current user's permissions. The
  plugin does not spawn a shell or execute user commands.
• License: MIT. No credentials, private prompts, transcripts, or Hook payloads
  are included.

Verification (2026-09-14, this head)

Catalog checkout:

• python3 scripts/validate.py → OK: 20 plugin(s) validated
• git diff --check clean; working tree clean
• plugins/zcode-plugin-langfuse/ byte-identical to the source build output
  (diff -r)

Source repository (tag v0.2.3):

• 29/29 tests pass, including a bundle-execution smoke (fail-open Stop hook
  returns {} and leaves zero state files) and a release-ZIP shape check
• npm audit --omit=dev --audit-level=high → 0 vulnerabilities
• Release workflow published the versioned ZIP + SHA-256 checksum

Runtime smoke (real ZCode session on the installed v0.2.3 build, verified via
the Langfuse API on 2026-09-14):

• ZCode Turn traces at 11:10:52Z and 11:19:41Z, both with
  release=0.2.3, assistant GENERATION observation present, and one turn
  additionally carrying a tool SPAN; local state cleaned up after each
  Stop. No credentials, prompts, tool payloads, or transcripts were exposed
  during verification.

Earlier updates (history)

• 2026-09-13: catalog-ready bundle layout replaced the hand-copied readable-JS
  snapshot (see below); superseded by the 2026-09-14 layout update above.
• This push also carried the zcode-plugin-langfuse rename (v0.2.0) and later
  the v0.2.1 → v0.2.3 syncs.

Update: catalog-ready bundle layout (2026-09-13) — superseded

This section described the payload/dist/hooks/entry.mjs entry point and the
artifacts/plugin-layout staging path of the v0.2.1 build. Both are replaced
by the 2026-09-14 official-template layout above; kept for history.