fix: seed an empty CHANGELOG.md so the first release is well-formed

[robinbowes]

The generated 0.1.0 notes on #1 end with a stray ## Changelog heading below the release entries. It is not a release-please bug — it is the seed content.

src/updaters/changelog.ts treats any pre-existing content as prior changelog body when it finds no version heading in it, demoting H1 to H2 and appending it below the new entry:

if(lastEntryIndex===-1){if(content){return`${this.header()}\n${this.changelogEntry}\n\n${adjustHeaders(content).trim()}\n`;}else{return`${this.header()}\n${this.changelogEntry}\n`;}}// adjustHeaders: content.replace(/^#(\s)/gm, '##$1')

So a seed of # Changelog becomes ## Changelog, parked under the first release's entries, where it then persists for the life of the file. An empty seed takes the second branch and is clean.

bootstrap seeded that heading into every generated project, so the defect propagated rather than being local to this repo — yo61/gh-release-stats carries the same artifact on main at line 36, from its own first release. yo61/unifictl does not, which is consistent with its changelog having been seeded differently.

Changes

• bootstrap writes an empty CHANGELOG.md instead of printf '# Changelog\n', with the reasoning recorded at the call site.
• This repo's own CHANGELOG.md is emptied so the pending 0.1.0 release is well-formed.

Not touched: the artifact already committed to gh-release-stats. Removing it there is a separate, cosmetic change to a shipped file.

Verification

Generated a project from this branch with ./bootstrap --defaults and confirmed its CHANGELOG.md is 0 bytes, then ran the updater's exact logic over both seeds:

seed '# Changelog' -> ends with a stray "## Changelog" (1 occurrence)
seed empty -> clean (0 occurrences)

./scripts/test-bootstrap.sh passes both the flat and DDD cases; shellcheck and shfmt -i 2 -bn -ci -sr clean.

[yo61-lastlight]

The PR removes the pre-seeded changelog heading from both the template and this repository, and the bootstrap reset now leaves CHANGELOG.md empty so release-please can generate the first changelog without demoting a seed heading. I found no blocking correctness or contract issues in the changed paths.