Docs: cover the CLI surface in the 0.15.0 release notes - #98

Summary

Adds the CLI's own surface to the 0.15.0 release notes. These changes were written for #97 but
landed a minute after it merged, so they missed the release section entirely — this carries them
onto main on top of it.

Documentation only. One file, +24 / −3.

Why this matters

The 0.15.0 notes describe the engine thoroughly and what a user actually types barely. Someone
reading them to find out what is new in the command line could not answer "what can I type now?"

Diffing the command surface, config keys and CLI flags against v0.14.3 — rather than working from
memory — turned up three gaps:

• /compact was never named. It appears only as a capability: "conversations can be compacted
  on demand." For a CLI release, the command is the feature. It now also says how it differs
  from /clear, since the two are easy to confuse and one of them is destructive.
• The retired planner / plannerEngine settings went unmentioned. /config refuses them now,
  because the workflow planner is the only execution path. Anyone who had set them would have met
  that with no warning in the notes.
• The startup banner change went unmentioned. It keeps a prerelease tag now, so v0.16.0-rc.1
  no longer displays as plain v0.16.0 — previously a test build and the release it was cut from
  looked identical, which is exactly when a stale binary is hardest to spot.

What changed

• A New & changed slash commands table, matching the format this file used at 0.11.0, so the
  whole surface is scannable in one place: /plan <goal>, /plan-resume, /plan-discard,
  /compact, and the /config keys that are gone.
• The compaction entry names /compact and contrasts it with /clear.
• Two new Changed entries for the retired settings and the banner label.
• A closing line noting MandoCode offers Resume or Discard for an unfinished plan at startup, so
  recovering after a crash does not require remembering a command.

Scope and risk

Low. Markdown under docs/, inside the already-dated [0.15.0] section. Nothing compiles
from it.

The one thing worth a reviewer's eye is that this edits a released section. The version is dated
but not yet tagged and not on NuGet, so the notes are still accurate to describe what will ship. If
you would rather freeze [0.15.0] as merged and carry these into 0.15.1, that is a reasonable call
and this can be retargeted.

Verification

• Command surface diffed against v0.14.3: four commands added (/plan, /plan-resume,
  /plan-discard, /compact), none removed. No CLI flags changed either way. Two config keys
  added, both as refusals.
• All four new commands now appear in the notes — confirmed by grep, not by eye.
• Verified the reassuring claim before making it: the notes say an existing config file carrying
  planner is not an error. Config reads do not set UnmappedMemberHandling, so System.Text.Json
  ignores unknown keys rather than throwing. Also confirmed strictPlanVerification is gone from
  src/ entirely, not merely undocumented.
• Entry count checked mechanically: 32 before the release roll, 35 after this lands. The two
  that appear to vanish are the compaction and migration entries, both reworded in place.

Not covered: no test run — nothing here is code, and the 733/733 figure in the Test coverage
section is unchanged and was measured on the release branch. Whether each pre-existing entry still
describes its feature accurately was settled in #96 and is untouched here.

Note on history

The commit is the one written for #97. It is unchanged apart from being rebased onto the merge.
#97's body was edited to describe this work after it had already merged, so that description does
not match what #97 actually contains — this PR is where it really lands.