fix(opencode): use symlinks for credentials in isolated summarization env

[OrnelasD-Rogers]

Problem

The OpenCode capture daemon (capture-daemon.py) creates an isolated environment
for opencode run summarization to prevent recursive plugin loading. However,
the isolation was incomplete:

• auth.json (provider credentials) was not available in the isolated env
• opencode.json was copied once on first run, so config changes were never
  picked up
• This prevented opencode run from using small_model configurations that
  rely on credentials registered with the OpenCode CLI

Solution

Replace file copying with symlinks for all required files in the isolated
environment:

File	Symlink target
~/.config/opencode/opencode.json	.../opencode/opencode.json
~/.local/share/opencode/auth.json	.../data/opencode/auth.json

Benefits:

• ✅ Provider credentials always available (auth.json)
• ✅ Config changes reflected immediately (symlinks, not stale copies)
• ✅ Still fully isolated (XDG_CONFIG_HOME + XDG_DATA_HOME separate)
• ✅ No recursion loop (daemon and subprocess use different SQLite DBs)

Files changed

• plugins/opencode/scripts/capture-daemon.py: Rewrite ensure_isolated_config()
  to use symlinks; add _ensure_symlink() helper
• docs/platforms/opencode/how-it-works.md: Update architecture diagram and
  isolation explanation to reflect symlink approach

Testing

Tested locally:

• Daemon starts successfully and creates correct symlinks
• opencode run in isolated env resolves models configured in small_model
• Summarization output appears as bullet points in .memsearch/memory/YYYY-MM-DD.md