chore: prepare release v0.6.0

[franciscojavierarceo]

Summary

Prepare the v0.6.0 release by:

• updating all workspace crate and published dependency versions to 0.6.0;
• promoting the accumulated Unreleased notes into a complete v0.6.0 changelog covering the changes since v0.5.0;
• updating the build-only Python wheel workflow and installation documentation for 0.6.0; and
• fixing the crate release dry run so it can validate agentic-server before agentic-server-core 0.6.0 exists on crates.io, with regression coverage and CI path triggers for release-workflow changes.

Test Plan

• cargo check --workspace
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• Python package suite: 113 passed, 3 skipped
• python3 scripts/tests/agentic-cli-e2e-test.py
• AGENTIC_API_RELEASE_VERSION=0.6.0 scripts/validate-python-release-version.sh
• cargo publish --dry-run --allow-dirty -p agentic-server-core
• cargo package --list --allow-dirty -p agentic-server
• mkdocs build --strict
• pre-commit run --all-files
• Independent pre-landing review: no findings

The Rust workspace suite passed with 0 failures. PostgreSQL integration tests remain repository-declared ignored tests because they require an isolated TEST_POSTGRES_URL.