docs: clean up AGENTS.md and append session log

[thepeak99]

Summary

Cleans up the contributor guide and appends a session log.

AGENTS.md — +81 / -6

• Adds an Install for agents section: the 4 release asset names, per-platform curl / Invoke-WebRequest download commands, the mail-cli install-skill step that materializes skill/ into ~/.agents/skills/mail-cli, the gh release download shortcut for gh users, and the cargo build --release + install-skill path for source builders. Documents that install-skill deliberately reads no MAILCLI_* env vars (safe to run before account config).
• Frames the file as designed for both humans and AI coding agents.
• Adds src/lib.rs, src/search.rs, and tests/ (gated GreenMail integration) to the project tree.
• Replaces stale No history yet / Integration (planned) lines with the current state (v0.1.0 released; integration tests gated on MAILCLI_IT=1).

memory.md — +35 session-log entries

• Entry for the v0.1.0 release (CI branch ci/add-github-actions merged via PR Add GitHub Actions CI + release pipelines #1; release.yml four-target matrix; tag v0.1.0 on 89a0b3a).
• Entry for this cleanup session, including the gh CLI sandbox workaround (gh auth token + curl to api.github.com; raw curl is ~0.17s while gh api/gh pr time out).

Test coverage

Docs-only — no source changes. cargo fmt, cargo clippy --all-targets, and cargo test remain green at 89a0b3a (the parent commit); this PR touches no Rust files.

Checklist

• cargo test clean (N/A — docs only)
• cargo clippy --all-targets clean (N/A — docs only)
• Diff is small and focused