fix: SuccessResult ignores -I, so ssky delete -I prints prose instead of the URI

[mkyutani]

Part of #72. Found while writing docs/OUTPUT_FORMATS.md (#76).

Problem

SuccessResult.print() only branches on JSON vs non-JSON:

if format in ('json', 'simple_json'):
    content = self.to_json()
else:
    content = str(self)

So id, text, and long all collapse to the human message. delete is the only
command returning this type, which means:

$ ssky delete at://…/app.bsky.feed.post/abc -I
Post deleted successfully

-I is the format whose contract is "identifier only" — it is what pipelines read. Every
other result type honours it. Getting prose here breaks the composition this project is
built around:

ssky search old-stuff -I | xargs -n1 ssky delete -I   # emits prose, not URIs

Work

• -I should emit the affected identifier (the deleted post's at://… URI, which
  SuccessResult already carries in data["deleted"])
• Decide -T and -L: -T as the message is defensible; -L should probably show the
  operation and its target
• Consider whether SuccessResult should carry a declared identifier field rather than
  each command inventing a data key, since more mutating commands are coming (feat: Add like / unlike commands #87)

Acceptance criteria

• ssky delete <uri> -I prints the URI and nothing else
• Behavior specified in docs/OUTPUT_FORMATS.md and the divergence row removed
• Test covering -I on a mutating command