Docs: record the 0.15.0 work missing from the changelog

[DevMando]

Summary

An audit of the 0.15.0 changelog against every pull request merged since v0.14.1 found three
user-visible changes that shipped without an entry. This adds them, so they are announced at
release rather than discovered by users.

Documentation only — no code, no behaviour change.

What was happening

The [Unreleased] section is what the release notes get written from. Thirty-five pull requests
have merged into this generation, and three of them left no trace in it — including a command
users can type. At release they would simply have gone unmentioned.

What is new

• /compact and the persisted compacted context. Desktop gained on-demand compaction, and the
  compacted context is saved so it survives a restart. /clear was reworded at the same time to
  state plainly that it wipes context completely, so the two commands are not confused. None of
  this was recorded.
• The deletion fix for the Snapshots, History, and Notes panels. Deleting an item used to
  rebuild the whole list, so surviving cards slid back up and expanded groups collapsed — losing
  your place mid-cleanup. The fix shipped; the entry did not.
• High-contrast notice cards. Status, plan, approval, and recovery notices are drawn as cards
  rather than plain text so they stay readable over a chat background at high image opacity. This
  belongs with the other readability entries in this release, which already cover the token metric,
  secondary text, and the empty state.

Scope and risk

Low. One markdown file. Nothing compiles from it and no code path reads it.

The risk is editorial: whether each entry matches what actually shipped. All three were written
from the merged PRs rather than from memory, but the /compact entry is the one most worth a
second read, since it is the only one describing a command whose exact wording users will compare
against what they see in the app.

Two things this deliberately does not add, called out so the omissions are a decision rather
than an oversight:

• Drop the verification-unavailable prompt with strict plan verification #51 (dropping the verification-unavailable prompt) removed unreachable code only — its own
  PR states no user-visible behaviour is lost, so there is nothing to announce.
• Improve agent context controls and notice readability #43's smaller UI polish — the list-style Files icon, and the composer placeholder naming the
  active agent — is below the bar the rest of this changelog sets. Say the word and I will add it.

Verification

• Every merged PR from v0.14.1..main (Document the release process in RELEASING.md #35 through Feature: agents can address each other by name #69) was listed and matched against the
  [Unreleased] section one by one; these three are the entries with no corresponding text.
• Confirmed the file's existing formatting convention still holds — every ### heading preceded
  by a blank line — since the insert points sit on section seams.
• The diff is +12 lines, 0 deletions: nothing existing was reworded or dropped.

Not covered: no build or test run, because nothing here is code. The suite was green at 489
passing on the parent branch. Whether each entry describes its feature accurately needs a human
read.

Dependency

Stacked on #70 (Midnight Ramen background), which also edits CHANGELOG.md. Merge #70 first;
this branch contains its commit and would otherwise conflict. Reviewing just the second commit
(a2b0259) shows this change on its own.

The paired engine audit is DevMando/MandoCode#96, which found seven gaps on that side —
including that per-step verification by a second model was removed without a changelog entry.