cleanup: drop pre-v0.1 migration scaffolding + legacy-behavior refs

banger hasn't shipped a public release — every "legacy", "pre-opt-in",
"previously", "migration note", "no longer" reference in the tree is
pinning against a state no real user's install has ever been in.
That scaffolding has weight: it's a coordinate system future readers
have to decode, and it keeps dead code alive.

Removed (code):
- internal/daemon/ssh_client_config.go
    - vmSSHConfigIncludeBegin / vmSSHConfigIncludeEnd constants and
      every `removeManagedBlock(existing, vm...)` call they enabled
      (legacy inline `Host *.vm` block scrub)
    - cleanupLegacySSHConfigDir (+ its caller in syncVMSSHClientConfig)
      — wiped a pre-opt-in sibling file under $ConfigDir/ssh
    - sameDirOrParent + resolvePathForComparison — only ever used
      by cleanupLegacySSHConfigDir
    - the "also check legacy marker" fallback in
      UserSSHIncludeInstalled / UninstallUserSSHInclude
- internal/store/migrations.go
    - migrateDropDeadImageColumns (migration 2) + its slice entry
    - dropColumnIfExists (orphaned after the above)
    - addColumnIfMissing + the whole "columns added across the pre-
      versioning lifetime" block at the end of migrateBaseline —
      subsumed into the baseline CREATE TABLE
    - `packages_path TEXT` column on the images table (the
      throwaway migration 2 dropped it, but there was never any
      reader)
- internal/daemon/vm.go
    - vmDNSRecordName local wrapper — was justified as "avoid
      pulling vmdns into every file"; three of four callers already
      imported vmdns directly, so inline the one stray call
- internal/cli/cli_test.go
    - TestLegacyRemovedCommandIsRejected (`tui` subcommand never
      shipped)

Removed / simplified (tests):
- ssh_client_config_test.go: dropped TestSameDirOrParentHandlesSymlinks,
  TestSyncVMSSHClientConfigPreservesUserKeyInLegacyDir,
  TestSyncVMSSHClientConfigNarrowsCleanupToLegacyFile,
  TestSyncVMSSHClientConfigLeavesUnexpectedLegacyContents,
  TestInstallUserSSHIncludeMigratesLegacyInlineBlock, plus the
  "legacy posture" regression strings in the remaining happy-path
  test; TestUninstallUserSSHIncludeRemovesBothMarkerBlocks collapsed
  to a single-block test
- migrations_test.go: dropped TestMigrateDropDeadImageColumns_AcrossInstallPaths,
  TestDropColumnIfExistsIsIdempotent; TestOpenReadOnlyDoesNotRunMigrations
  simplified to test against the baseline marker

Removed (docs):
- README.md "**Migration note.**" blockquote about the SSH-key path move
- docs/advanced.md parenthetical "(the old behaviour)"

Reworded (comments):
- Dropped "Previously this file also contained LogLevel DEBUG3..."
  history from vm_disk.go's sshdGuestConfig doc
- Dropped "Call sites that previously read vm.Runtime.{PID,...}"
  from vm_handles.go; now documents the current contract
- Dropped "Pre-v0.1 the defaults are" scaffolding in doctor_test.go
- Dropped "no longer does its own git inspection" phrasing in vm_run.go
- Dropped the "(also cleans up legacy inline block from pre-opt-in
  builds)" aside on the `ssh-config` CLI docstring
- Renamed test var `legacyKey` → `existingKey` in vm_test.go; its
  purpose was "pre-existing authorized_keys line," not banger-legacy

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config_test.go

@@ -41,9 +41,9 @@ func TestLoadDefaultsResolveFirecrackerAndGenerateSSHKey(t *testing.T) {
 			t.Fatalf("stat %s: %v", path, err)
 		}
 	}
-	legacyKey := filepath.Join(configDir, "ssh", "id_ed25519")
-	if _, err := os.Stat(legacyKey); err == nil {
-		t.Fatalf("key was also generated at legacy path %s; config.Load must not write under ConfigDir/ssh anymore", legacyKey)
+	forbiddenKey := filepath.Join(configDir, "ssh", "id_ed25519")
+	if _, err := os.Stat(forbiddenKey); err == nil {
+		t.Fatalf("key was also generated at %s; config.Load must not write under ConfigDir/ssh", forbiddenKey)
 	}
 	if cfg.DefaultImageName != "debian-bookworm" {
 		t.Fatalf("DefaultImageName = %q, want debian-bookworm", cfg.DefaultImageName)
@@ -72,8 +72,8 @@ func TestLoadSSHKeyPathExpandsHomeAnchored(t *testing.T) {
 
 // TestLoadNormalizesAbsoluteSSHKeyPath pins filepath.Clean behaviour
 // for configured paths: trailing slashes and duplicate slashes are
-// flattened so downstream comparisons (e.g. sameDirOrParent) don't
-// see two spellings for the same path.
+// flattened so downstream path comparisons don't see two spellings
+// for the same path.
 func TestLoadNormalizesAbsoluteSSHKeyPath(t *testing.T) {
 	cases := []struct {
 		name string