feat(platform): namespace Developer Platform commands (DEVX-621)

[smukherjee-godaddy]

Summary

• add the implementation-ready plan and the gddy platform command namespace
• replace the root-level application, actions, and webhook command families
• make gddy platform app canonical, retaining gddy platform application as a nested alias only
• update help, next actions, scope references, feature-flag documentation, and README migration guidance
• add command-tree, help, alias, legacy-root rejection, and fail-closed-auth regression coverage

Migration

• gddy application … → gddy platform app …
• gddy actions … → gddy platform actions …
• gddy webhook … → gddy platform webhook …

Verification

• cargo fmt --check
• cargo check
• cargo clippy -- -D warnings
• cargo test (362 passed)
• manual Experimental-mode tree/help checks using PROD_MIN_STAGE=experimental

Jira: https://godaddy-corp.atlassian.net/browse/DEVX-621