feat(configure): add project_id field to config

[tvt286]

Summary

VNG Cloud APIs embed the user's project UUID in many URL paths (e.g. /v2/{projectId}/networks). Adding project_id to the CLI config lets downstream tools (greenode-mcp-server, future SDKs) read a cached value instead of calling /v1/projects every session.

• Scope: per-profile field stored in ~/.greenode/config alongside region and output. Not a secret — lives in config, not credentials.
• Resolution order: GRN_DEFAULT_PROJECT_ID env var > config file value > empty (downstream tool can auto-detect)

Changes

• grn configure: wizard prompts for Project ID (blank = skip)
• grn configure list: includes project_id with source column (env / config-file)
• grn configure get project_id
• grn configure set project_id <value>
• GRN_DEFAULT_PROJECT_ID env var override
• Profiles work as-is (each profile has its own project_id)
• Docs updated (README, docs/configuration.md)

Test Plan

• grn configure set project_id pro-xxx persists to ~/.greenode/config
• grn configure get project_id returns value
• grn configure list shows field + source (env vs config-file)
• GRN_DEFAULT_PROJECT_ID override takes priority over file
• go vet + go build clean
• Setting empty value clears the field
• Manual: wizard end-to-end with a fresh profile
• Downstream: greenode-mcp-server reads this field (follow-up PR)

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