Prepare v0.3.0 release from upstream main

[tpmeredith]

The latest packaged release, v0.2.9, predates the merged empty-thread fix in #21 and the subsequent inbox, reliability, and optional v2 work. This proposes v0.3.0 / macOS build 19, adds upgrade notes, and makes release builds consistently use the requested tag and its resolved commit.

• Fix manual releases selecting the workflow branch name instead of the requested tag. Validate tag existence, mainline ancestry, and macOS version before building; use the same commit and version for every artifact.
• Require dependency verification and Go normal/race tests before publication. Disable workspaces, private-module overrides, and restored Go caches for release builds; resolve modules through the public proxy/checksum database.
• Update release notes/checklist and the documented Go requirement. The dual-SIM symptoms in [Bug] Critical issues when migrating from Google Messages #158 are not claimed fixed.

Source and dependencies

Based directly on upstream main c7d7445474ec75662659377da984cb862c62fca0, with only this release-preparation commit added. Dependency manifests and the Dockerfile are unchanged. The only module replacement remains the maintainer's public MaxGhenis/gmessages fork at 0e43542dfa0e0b97e410f185a5842e8740106099; this PR is independent of #179 and requires no contributor fork, local workspace override, or unmerged integration commit.

Validation

• GitHub CI run 34478503088 passed all six checks at head 32716e5b4cde475fef7229bc944ff4b0066c4681: Release Metadata, Go Test, Go Race, Web E2E, Site Build, and macOS App Build.
• Fresh, credential-free container: all 88 modules resolved/verified publicly; full Go suite passed, 65.0% coverage.
• Full Go race suite and go vet ./... passed.
• Playwright: 91 passed, 9 existing skips, synthetic fixtures only.
• Eight release-metadata regression tests and actionlint/ShellCheck passed.
• All four Linux/macOS x86-64/ARM64 CLI targets built; the packaged archive checksums verified.
• Exact PR-head Git archive built with the checked-in Dockerfile. The resulting v0.3.0 image passed UI, empty-inbox, diagnostics, MCP version, and restart checks with networking disabled, no published ports, and a disposable data volume.

Before publishing

Keep this PR in draft for maintainer release review. Native macOS compilation, tests, and dev/release packaging passed in CI; Apple signing/notarization and authorized live pairing/send/receive, including dual-SIM behavior, remain to be validated. Existing installations were not changed. No release tag or downloads have been published; a maintainer must merge the PR and tag the approved commit to trigger publication.