Prepare AgentP v0.1.0 for its first release

[yoelrosenthal]

Changes

• Replace placeholder changelog notes with the first-release feature list dated September 18, 2026.
• Document the release checklist and changelog drafting process, and mention the included favorite podcasts.
• Publish reviewed changelog entries as GitHub release notes and reject missing, empty, or duplicate entries.
• Replace the retired Intel macOS runner and correct the minimum Rust version to 1.88.

Verification

• cargo test --all --locked: 112 tests passed.
• cargo fmt --all -- --check: passed.
• cargo clippy --all-targets --all-features --locked -- -D warnings: passed.
• cargo +1.88.0 check --locked: passed.
• cargo package --locked --allow-dirty: packaged and compiled successfully.
• Release workflow YAML parsed and five release-note extraction cases passed.
• git diff --check: passed.

Cross-platform release builds and publishing credentials remain unverified. This PR does not tag or publish a release.

---

Summary by cubic

Prepares AgentP v0.1.0 for its first public release: finalizes the changelog feature list, documents the release process for maintainers, and switches release notes to come from the reviewed changelog.

• Release notes are now extracted verbatim from the matching CHANGELOG.md entry and fail on missing, empty, or duplicate entries; git-cliff is no longer used in the release workflow.
• Intel macOS builds move from the retired macos-13 runner to macos-15-intel.
• Minimum Rust version is raised to 1.88, and the documented Cargo install and build commands now use --locked.
• README adds a maintainer release checklist and changelog drafting guide, and notes that AgentP ships with demo podcasts.

^{Written for commit 4b5534c. Summary will update on new commits.}

Note

Prepare release workflow and docs for AgentP v0.1.0 first release

• Rewrites the release flow in release.yml so the release.verify job extracts release notes from the tagged CHANGELOG.md section and uploads RELEASE_NOTES.md; release.create-release consumes that artifact instead of running git-cliff locally
• Constrains the Homebrew and Scoop update jobs to download only agent-p-* artifacts, and moves the x86_64-apple-darwin build to the macos-15-intel runner
• Bumps the minimum supported Rust version from 1.87 to 1.88 in Cargo.toml and updates README.md build instructions accordingly
• Adds a maintainer release guide at docs/releasing.md and rewrites the v0.1.0 CHANGELOG entry with the release statement and Added section
• Behavioral Change: the release now fails if the tag does not have exactly one nonempty dated CHANGELOG.md entry; MSRV is now 1.88

^{Macroscope summarized 5efe3db.}

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