Improve Pi compatibility and abstract harness config resolution

[kadams54]

Summary

• add Pi marketplace parity entries in .agents/plugins/marketplace.json (including han, han-documentation, han-research, and han-linear)
• make personal config path resolution harness-agnostic via AGENT_CONFIG_DIR with fallback to CLAUDE_CONFIG_DIR then ~/.claude
• update skill probes, shared config rules, and docs to use the new config-root contract while keeping Han config at .han/config.md
• add lightweight compatibility checks (check:pi-compat, advisory check:docs-host-wording, and aggregate check:compat)
• add tiny environment setup guidance in README and configuration docs

Validation

• npm run check:compat

Notes

• docs host-wording check is advisory (non-blocking) to keep friction low
• Pi compatibility check validates a required plugin subset (not strict 1:1 marketplace equality) to reduce brittleness

[mxriverlynn]

@kadams54 this is great! 🥳 i especially like the AGENT_CONFIG_DIR env var addition - making that configurable is a huge win and makes this so much more flexible!

one question about that: is AGENT_CONFIG_DIR the right name for this? or should we go with HAN_CONFIG_DIR? or ?

i was looking at Pi briefly, and saw that it uses PI_CODING_AGENT_DIR as the equivalent of CLAUDE_CONFIG_DIR ... not saying any particular option is right or wrong. mostly just wondering if there's a good reason to use one over the other.