fix(hermes): install memory provider in user plugin dir

[siolyn]

Summary

Fixes #167.

The Hermes installer previously linked memory_tencentdb only into the mutable Hermes checkout tree:

$HERMES_AGENT_DIR/plugins/memory/memory_tencentdb

If the Hermes checkout is replaced during an update, that symlink can disappear while memory.provider: memory_tencentdb remains configured, causing TencentDB memory to silently detach.

This PR keeps the checkout link for compatibility and also installs a durable user-scope memory provider link:

$HERMES_HOME/plugins/memory_tencentdb

Hermes memory provider discovery scans $HERMES_HOME/plugins/<provider_name>/, so this is the path that survives checkout replacement.

Changes

• Add user-scope Hermes memory provider symlink.
• Keep existing checkout-tree symlink for compatibility.
• Validate source provider files before creating any links.
• Refuse to overwrite non-symlink targets.
• Print readlink results in the install summary.

Verification

• bash -n scripts/install_hermes_memory_tencentdb.sh
• git diff --check
• Smoke-tested install through Step 2.5 with temporary HERMES_HOME / HERMES_AGENT_DIR
• Verified both plugin paths are symlinks pointing at the source provider and both expose plugin.yaml

Note: the local smoke-test run reached the symlink step successfully, then stopped at the pre-existing /etc/profile.d sudo write because this macOS test environment requires an interactive sudo password.

[siolyn]

Superseded by #170 using a non-codex branch prefix.

[Maxwell-Code07]

Received, we will follow #170.