Release v0.19.0: experimental OpenCode v2 support

[oribarilan]

Summary

Prepare a normal v0.19.0 release after #82, which is already squash-merged into main.

• Bump package.json and src/version.ts together to 0.19.0.
• Move the completed changes into the dated 0.19.0 changelog section and update comparison links.
• Begin the gradual rollout of OpenCode v2 support as experimental. Future maintenance will move to v2 only; this release still includes v1 compatibility, with no removal date set.
• Recommend v0.18.1 as the legacy v1 pin. V1 users can opt into v0.19.0 for shared fixes, or keep the older version and disable update notifications.
• Replace v2 installation/options/leader examples with the v0.19.0 Git tag. Keep the recommended v1 examples pinned to v0.18.1.
• Clarify v2 rollback: remove the plugin entry rather than installing the v1-only v0.18.1 release.

This PR changes release metadata, documentation, and the version constant only. It does not change the Vim engine or host adapter behavior from the merged #82 implementation.

Validation

• just check: lint clean, 337 Bun tests passed.
• just compat-unit: 16 Python tests passed.
• git diff --check: passed.
• Version constants, changelog links, v1 legacy pins and v2 release refs checked for consistency.
• The prepared vimcode-0.19.0.tgz was tested through the installed-package cache on macOS ARM64. Both runs verified source files and the manifest byte-for-byte:

Host	Cold install	Warm install	Exact-state checks passed	Named known gaps
OpenCode 1.18.33	Passed	Passed	39	2
OpenCode 2.0.15	Passed	Passed	48	2

The remaining probes are snapshot-redo and tab-inner-word-offset, both inherited gaps. Tests used isolated HOME/XDG state, a private tmux socket, synthetic text and clipboard stubs; no model requests or permission approvals were made.

A clean-dependency typecheck retains the eight baseline errors documented in #82. No new type diagnostics were introduced by the version bump.

Publication gates

This PR does not create or publish a release.

After approval and green CI:

1. Squash-merge this release PR.
2. Create and push tag v0.19.0 on the release commit.
3. Verify the final Git-tag installation on the pinned v1/v2 hosts.
4. Publish a normal GitHub release using the changelog notes.

The v0.19.0 tag does not exist yet, so its installation examples become usable only after that gated tag step. Earlier Git-install validation in #82 used the pinned implementation commit, not this future tag. The dated changelog heading should match the actual publication date if release is delayed.

Validation remains bounded to OpenCode 1.18.33/2.0.15 on macOS ARM64; experimental v2 support is not an all-versions, all-platforms or full-parity promise.