fix(kyberforge): bridge apm content to Claude Code's flat plugin discovery

Claude Code's (and Copilot's) native plugin installer has zero awareness of
.apm/ nesting -- it convention-scans only flat skills/, agents/, commands/,
hooks.json at each plugin's root. Confirmed via strings on the installed
claude binary and live installs of git@holocron/gitea@holocron/kyberforge@
holocron, all reporting Skills(0) Agents(0) Hooks(0) post ADR-0015's apm
conversion. Root cause (apm_cli/core/plugin_manifest.py): apm's plugin.json
compiler deliberately strips skills/agents/commands keys, assuming the host
already auto-discovers those convention directories -- it has no model of
.apm/ being host-visible at all. Separately, apm's own bundle exporter
(apm_cli/bundle/plugin_exporter.py, behind `apm pack --format plugin`)
implements the correct .apm/ -> flat mapping, but only ever targeted
build/<name>-<version>/, a path nothing in marketplace.json's source: points
at.

scripts/sync-plugin-content.sh wraps that bundle exporter and copies its
agents/, skills/, commands/, instructions/, extensions/, and merged
hooks.json back into each plugin's own root as a second tracked
compiled-output category -- same governance status as
.claude-plugin/plugin.json: generated from .apm/, never hand-edited. tests/
subdirectories are excluded from the mirror (dev fixtures, not host-visible
runtime content; several hardcode a relative repo-root walk-up sized for the
.apm/-nested depth, which breaks when duplicated one level shallower).
Applied for real across all 6 plugins and verified two ways: `claude plugin
validate --strict` passes on every real plugin directory, and a live
`claude --plugin-dir <path> -p "list skills/agents"` behavioral test
confirms content is now actually discovered.

Also, from the same issue #90 review round:
- scripts/check-manifests.sh pointed at each plugin's root-level plugin.json
  (checking skills/hooks/mcpServers/agents pointer fields) -- that file was a
  stale near-duplicate of .claude-plugin/plugin.json nothing else read or
  wrote, now deleted across all 6 plugins. check-manifests.sh is rewritten to
  validate .claude-plugin/plugin.json instead, and drops the pointer-field
  checks entirely (nothing to check -- those fields are correctly absent by
  design). Content-presence drift is now check-plugin-content-sync's job, a
  new pre-push hook wired in .pre-commit-config.yaml.

docs/adr/0017 records the root cause and decision in full, including two
rejected alternatives (patching plugin.json's path fields directly -- apm's
compiler strips them on every run; pointing marketplace.json at apm pack's
build/ output -- a version-suffixed non-source directory nothing can install
from without an extra build step). ADR-0015 and CONTEXT.md are updated to
point at it.

Refs: #90

plugins/kyberforge/skills/agent-author/assets/templates/copilot.agent.md.template

@@ -0,0 +1,59 @@
+---
+<!-- GitHub Copilot CLI agent definition (CLI format — path: .github/agents/<name>.agent.md)
+     File extension MUST be .agent.md — a plain .md file is not picked up by Copilot CLI.
+     Fill in all FILL IN: placeholders. Remove or uncomment optional fields as needed.
+     Body length limit: 30,000 characters — content beyond this is silently truncated.
+     Delete template comments before shipping.
+
+     NOTE: This template is for the CLI format. The cloud/IDE format (path: .github/copilot/agents/<name>.md,
+     extension: .md) supports additional fields: target, user-invocable, disable-model-invocation, mcp-servers.
+     Do not add those fields here — they are silently ignored by the CLI runtime. -->
+
+name: AGENT_NAME
+<!-- Required. Kebab-case identifier. Home-directory version wins on name collision. -->
+
+description: FILL IN: Action-first description of what this agent does and when to invoke it.
+<!-- Required. Used by the runtime for automatic agent selection — quality matters.
+     Start with a verb: "Reviews...", "Analyzes...", "Generates..."
+     Example: "Reviews pull request diffs for security issues." -->
+
+<!-- tools: ["read", "search", "edit"]
+     Optional. Array of tool names. Omit = all available tools. [] = no tools.
+     Copilot tool aliases (use these, not Claude Code names):
+       execute   — run shell commands (aliases: shell, Bash, powershell)
+       read      — read file contents (aliases: Read, NotebookRead)
+       edit      — modify files (aliases: Edit, MultiEdit, Write, NotebookEdit)
+       search    — search files (aliases: Grep, Glob)
+       agent     — invoke sub-agents (aliases: custom-agent, Task)
+       web       — web search and fetch (aliases: WebSearch, WebFetch)
+     For MCP tools: "server-name/tool-name" or "server-name/*" -->
+
+<!-- model: claude-sonnet-4-5
+     Optional. Model to run this agent on.
+     Cloud/IDE-only fields (target, user-invocable, disable-model-invocation, mcp-servers)
+     are not valid in this CLI format — use the .github/copilot/agents/<name>.md path for those. -->
+
+<!-- source_keys:
+       - slug-name
+     Development-only. Add when research sources informed this agent (slugs must match sources.md at the plugin root).
+     Omit when no research was used. Not a Copilot runtime field — silently ignored. -->
+
+<!-- DO NOT include these Claude Code-only fields:
+       maxTurns, isolation, memory, permissionMode, effort, hooks, mcpServers -->
+---
+
+FILL IN: System prompt body. Should match the Claude Code version — the agent's task is the same across providers.
+
+You are a FILL IN: role description. When invoked, FILL IN: primary action.
+
+## Inputs
+
+FILL IN: What inputs does this agent expect? (files, context, parameters)
+
+## Process
+
+FILL IN: Steps the agent takes. Be specific about ordering if it matters.
+
+## Output
+
+FILL IN: What does the agent produce? Format, location, structure.