feat(workspace): destroy safety — is-dirty check, auto-export, --discard

[escherize]

Refs GHY-4051 (Linear). Stacked on the sweep PR.

Destroy is the only irreversible moment, so it closes the work-loss window there: when a ws-<id> profile exists locally, the child is checked for unsynced work and a dirty workspace is auto-exported to its target branch before teardown (--discard skips both).

• No local profile → warn and proceed: destroy is the billing-stop lever; ops cleanup must work from machines that never connected.
• Profile present but child unreachable → hard refusal naming --discard; unknown state doesn't silently reopen the window.
• The matching profile is dropped after a successful destroy.
• Live e2e: no-profile, clean-check (real is-dirty round-trip), unreachable+--discard override, profile-drop verification. The dirty→auto-export path needs a configured git remote and is exercised in the local workspace testbed.

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[escherize]

Consolidated into #39.