Add railway flag command for feature flag management

[futurepastori]

Summary

Adds CLI support for Railway Feature Flags — define flags, set default values, attach targeting rules, delete flags, and inspect registered flags for a workspace.

• railway flag is the primary command
• Set defaults with railway flag set <name> <value>
• Targeting rules use the same set command with --when
• CEL-style --when subset for common predicates, plus raw JSON as an escape hatch
• Tree-style list output shows flag values and rule predicates; --full includes rule IDs and empty rule sections
• Delete flags with railway flag delete <name>
• Remove rules with railway flag unset <name> --rule-id <id>
• Type safety — values are inferred (true, 42, "blue", JSON objects); explicit --type supported on set
• Re-type existing flags with railway flag set --force --type (clears rules when the type changes)

Owner scope defaults to the linked project's workspace (railway link), or can be set with --owner / RAILWAY_SIGNALS_OWNER.

Examples

railway flag list
railway flag list --full
railway flag set checkout.v2 true
railway flag set theme "blue"
railway flag set checkout.v2 true --when 'workspace_plan == "enterprise"'
railway flag set checkout.v2 true --when 'bucket(workspace_id) < 0.25'
railway flag delete checkout.v2
railway flag unset checkout.v2 --rule-id enterprise-on
railway flag set checkout.v2 "yes" --force --type string
railway flag list --json

--when supports a CEL-shaped subset for comparisons, &&, ||, !, receiver calls like source_repo.contains("miner"), list membership, and bucket rollouts. Raw JSON expressions are still accepted for full-fidelity use cases.

Test plan

• cargo fmt
• cargo test signals --quiet
• cargo test flag_list_tests --quiet
• cargo build
• railway flag list against a linked project with flags configured
• Create a flag, add a rule, verify with list --full and list --json
• Confirm type-mismatch error and --force --type re-type flow

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