/archon-upgrade locates the bundled Boxes through $CLAUDE_PLUGIN_ROOT, which is unset

The defect

docs/adr/0035-archon-upgrade-report.md line 91 says the command reads "the bundled YAMLs under $CLAUDE_PLUGIN_ROOT/.archon/workflows/" — and line 36 of the same ADR already records that this variable is unset inside the Bash tool. The command inherits the exact locate problem #383 just solved for /setup.

Found by #383's rewrite (2026-08-25, reported by its session); out of #383's scope, routed here.

The settled mechanism to adopt

#383 decided it and ADR-0036 (amended 2026-08-25) records it: read Claude Code's registry at ~/.claude/plugins/installed_plugins.json, match the entry on projectPath, take installPath — never search the version cache, which holds one directory per version ever installed.

One thing to settle while in the file

Two surfaces disagree about which Boxes the command reads: ADR-0035:91 says the bundled YAMLs (the plugin's own copies), while the plugin AGENTS.md doctrine says the ADR-0011 trap re-assertion reads "each installed .archon/workflows/unic-dlc-*.yaml" (the Consumer's copies). Decide which read each step intends — they answer different questions (what the release ships vs what this Consumer runs) — and make the command and the ADR agree.

Acceptance criteria

• commands/archon-upgrade.md states the registry-lookup mechanism for locating the plugin's installed directory, and stops rather than guessing when the registry has no matching entry.
• The bundled-vs-installed question above is decided and both surfaces (command + ADR-0035) say the same thing.
• ADR-0035 is amended per the plugin's dated-blockquote convention.

Context

• A real /archon-upgrade run against Archon 0.9.0 is imminent (the command's first proof run, tracked on #373) — its transcript is evidence for this ticket: note which step hits the unset variable and what the failure looks like.
• Related: #400 (the same command misreads a permission denial, needs-triage) — same proof run may evidence both.

Start with commands/archon-upgrade.md and docs/adr/0035-archon-upgrade-report.md; read ADR-0036 and the registry lookup from #383. Run the /archon-upgrade proof described in #373 and capture the unset-variable failure. Done means the command documents safe lookup and stopping behavior, the bundled-versus-installed read is decided, and both files agree with an amended ADR block.