fix(changelog): restore [Unreleased] heading on main + document the rule

[ThomasK33]

Summary

After the v0.2.0 release-prep commit (#102), ## [Unreleased] was renamed to ## [v0.2.0] - 2026-05-13 but a fresh empty ## [Unreleased] heading was not inserted above it. That left main with no Unreleased section, which breaks the Update Unreleased Changelog workflow that runs on every push to main:

× CHANGELOG.md exists but does not contain ## [Unreleased] or ## Unreleased; add an Unreleased section before running `communique generate HEAD --changelog`.

Failing run: https://github.com/coder/agent-tty/actions/runs/25795451618/job/75771464557

Changes

• CHANGELOG.md — insert an empty ## [Unreleased] heading above ## [v0.2.0] - 2026-05-13 so future post-release merges flow through the unreleased-changelog bot correctly.
• docs/RELEASE-PROCESS.md — new "`[Unreleased]` heading must stay on `main`" subsection. Spells out the rename-and-insert rule, explains that two separate workflows depend on the two different headings, shows the expected CHANGELOG.md shape after the release-prep commit, and instructs the maintainer to hand-edit CHANGELOG.md and amend before pushing the release-prep PR.

Why I didn't catch this during the v0.2.0 cut

Both workflows that depend on the CHANGELOG headings are well-designed but their requirements pull in opposite directions:

• Release Changelog (release-changelog.yml) keys off ## [v<version>] to skip its Communique pass on the release branch — this is why I deliberately renamed ## [Unreleased] → ## [v0.2.0] in the release-prep commit.
• Update Unreleased Changelog (update-unreleased-changelog.yml) keys off ## [Unreleased] to know where to insert post-merge entries on main.

The correct shape carries both headings at once on main, with [Unreleased] empty and [v<version>] carrying the finalized release notes. Doing the rename without inserting a fresh [Unreleased] satisfies the first workflow but breaks the second.

Future improvement (not in this PR)

scripts/release-prep.mjs could be taught to perform the rename + fresh-Unreleased-insert automatically when invoked with `--changelog ci`, removing the manual editing step entirely. Tracked as a follow-up.

Test plan

• `npm run format:check` clean
• CI on this PR
• After merge: confirm next push to main no longer fails the `Update Unreleased Changelog` workflow

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