docs(changelog): file the codemode single-ship entry under Unreleased and repair section spacing

[code-yeongyu]

Summary

Changelog-only. Prepares the [Unreleased] sections for the next release (2026.9.15-2), which ships #1683, #1694, #1700, #1702, #1707, #1720 and #1721 on top of v2026.9.15.

• packages/coding-agent/CHANGELOG.md: the codemode single-ship entry (build(codemode): ship codemode once from the on-disk package #1683, merged after the v2026.9.15 tag) sat inside the already released [2026.9.15] section; it moves to [Unreleased] > Changed. The [Unreleased] > Fixed block had lost the line break before ### Removed (...#1698)).### Removed) and carried double blank lines between entries; both repaired.
• packages/senpi-codemode/CHANGELOG.md: restores the blank line before ### Removed in [2026.9.15].

No entry text was rewritten. polish-ai-tells lint on the [Unreleased] prose: 0 hits, score 1.0 (the bullet-ratio metric is the changelog register).

QA & Evidence

• git diff shows moves and whitespace only; every entry keeps its issue link.

Related Issues

Refs #1656, #1695, #1696, #1698, #1507.

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Summary by cubic

Changelog-only: files the codemode single-ship entry under [Unreleased] and repairs section spacing in both changelogs so the upcoming 2026.9.15-2 release renders cleanly.

• Moves the codemode single-ship entry from the released [2026.9.15] section to [Unreleased] > Changed in packages/coding-agent/CHANGELOG.md.
• Restores the missing line break before ### Removed and removes double blank lines between entries in packages/coding-agent/CHANGELOG.md.
• Adds the missing blank line before ### Removed in packages/senpi-codemode/CHANGELOG.md.

No entry text was rewritten; the diff is moves and whitespace only.

^{Written for commit 3f79c83. Summary will update on new commits.}