feat: cli spec reconciliation part 2

[yarolegovich]

• Rename --polling-interval -> --poll-interval.
• Allow : as service parameter separator.
• In text mode print task resume hint for input and auth required states.
• Introduce --a2a-version which can pin protocol version to a specific value. The default is all supported transports.
• Fix events printed without wire one-of envelope in json mode.
• Fix jsonl being indented. Added --pretty for re-enabling indentation.

fixes #32 #35

[msampathkumar]

Reviewed this PR against the currently-filed issues. It implements two of them:

• feat: add --a2a-version to select the A2A protocol version #32 — introduces the --a2a-version flag and wires ProtocolVersion into transport registration (0.3 / 1.0), with tests.
• feat: print the exact resume command when a task pauses #35 — adds a copy-pasteable resume hint in text mode (formatResumeHint → a2a send --task-id <id> "<reply>") for INPUT_REQUIRED / AUTH_REQUIRED, with a test.

Attaching both here so they close when this merges.

One clarification so it isn't assumed covered: #36 is not addressed by this PR — internal/polling/polling.go is untouched, so the blocking-wait poll loop still doesn't stop on AUTH_REQUIRED (that's separate from printing the resume hint). #36 remains open.