Document the complete Base v1.9.0 release in CHANGELOG

[codeforester]

Goal

Update the dated v1.9.0 section of CHANGELOG.md so it accurately summarizes the work shipped in Base v1.9.0.

Background

The v1.9.0 milestone and release are complete, including the Base-owned immutable ecosystem compatibility BOM and validated Ubuntu 24.04/macOS 14 release matrix. The current changelog section records many implementation changes but omits the release-defining BOM and several shipped workspace safety and release-provenance changes. The edit currently exists only as an uncommitted local change on the already-merged PR #2086 branch and needs to land through the normal issue/PR process.

Scope

• Update only the dated 1.9.0 section in CHANGELOG.md.
• Record the immutable Base/base-cli/base-bash-libs/base-demo compatibility BOM and required platform validation.
• Summarize the shipped workspace apply/JSON/reporting, containment, redaction, delegated-run ownership, and release-provenance improvements.
• Preserve the existing Keep a Changelog structure and leave unrelated post-1.9.0 work out of this historical section.

Acceptance Criteria

• CHANGELOG.md has a dated [1.9.0] - 2026-09-08 section containing the release BOM and Ubuntu 24.04/macOS 14 validation.
• The section accurately summarizes the shipped workspace safety/consent, trust, runtime, and release-provenance changes without claiming deferred work shipped.
• No Unreleased entries or files outside CHANGELOG.md are changed.
• The issue is linked from the pull request and the pull request passes required hosted checks before merge.

Validation

• git diff --check
• Inspect the final CHANGELOG.md diff for scope and factual accuracy.
• Verify the merged pull request and the updated main file after hosted checks pass.

Non-Goals

• Do not change the already-published GitHub Release, milestone description, version files, or release artifacts.
• Do not add post-1.9.0 work to this historical release section.
• Do not change implementation code, tests, or other documentation.

Project Fields

• Status: Backlog
• Priority: P2
• Size: S
• Area: Docs
• Initiative: Adoption Polish

Agent Assignment

• Human first

Ownership

• Assignee: @codeforester