Delete dead code and stale fixtures, fix config set reindent (closes #70)

Remove the unused internal/models package (and its coverage-only test),
internal/cli/vaultik_snapshot_types.go (a second, dead SnapshotInfo that
shadowed the live type in internal/vaultik), and two fixtures
(test-config.yml, test/integration-config.yml) that use source_dirs and
other keys config.Config no longer has. All confirmed by grep against
origin/next; none has a production consumer.

config set now writes via yaml.NewEncoder with 2-space indent, matching
defaultConfigTemplate, so the first set no longer reindents the file it
promises to preserve. A test covers the round-trip.

Single commit rather than one commit per deletion, per task instruction.
test/integration-config.yml removed per the manager note on the issue.

Model: opus-4-8

test/integration-config.yml

@@ -1,24 +0,0 @@
-age_recipients:
-  - age1278m9q7dp3chsh2dcy82qk27v047zywyvtxwnj4cvt0z65jw6a7q5dqhfj  # sneak's long term age key
-  - age1ezrjmfpwsc95svdg0y54mums3zevgzu0x0ecq2f7tp8a05gl0sjq9q9wjg  # insecure integration test key
-source_dirs:
-  - /tmp/vaultik-test-source
-exclude:
-  - '*.log'
-  - '*.tmp'
-  - '.git'
-  - 'node_modules'
-s3:
-  endpoint: http://ber1app1.local:3900/
-  bucket: vaultik-integration-test
-  prefix: test-host/
-  access_key_id: GKbc8e6d35fdf50847f155aca5
-  secret_access_key: 217046bee47c050301e3cc13e3cba1a8a943cf5f37f8c7979c349c5254441d18
-  region: us-east-1
-  use_ssl: false
-  part_size: 5242880  # 5MB
-index_path: /tmp/vaultik-integration-test.sqlite
-chunk_size: 10MB
-blob_size_limit: 10GB
-compression_level: 3
-hostname: test-host