drawbridge up --patch-env: update existing .env file with drawbridge URLs

[radiosilence]

Problem

Currently --local b2c-spa generates a standalone .env.drawbridge that the user has to manually copy or source. But most projects already have a .env file with their own config (third-party keys, feature flags, etc.) — they don't want to replace it wholesale, they just need the drawbridge-managed URLs patched in.

Proposed solution

drawbridge up --local b2c-spa --patch-env ../app-b2c-spa/.env

Behaviour

1. Parse the target .env file
2. For each env var in the service's drawbridge config (env: block), check if it exists in the target file
3. Update matching keys with the drawbridge values (e.g. B2C_API_GATEWAY_URL gets rewritten from the staging URL to https://api.b2c.dev.local/graphql)
4. Append any keys from drawbridge config that don't exist in the target file (commented with # Added by drawbridge)
5. Write the patched file back

Considerations

• Should create a .env.drawbridge.bak backup before patching
• Should be idempotent — running twice doesn't duplicate entries
• Need to handle comments and blank lines in the original .env gracefully
• Could mark patched lines with a trailing comment # [drawbridge] so they can be identified/reverted
• drawbridge down or a drawbridge unpatch command could restore from backup
• Should warn if a key exists in both the original and drawbridge config with different values (so user knows what's being overwritten)

Example

Before:

B2C_API_GATEWAY_URL=https://b2c-api-gateway-eng-mint.dev.fresha.io/graphql
REDIS_HOST=localhost
FACEBOOK_APP_ID=630728900463808

After --patch-env:

B2C_API_GATEWAY_URL=https://api.b2c.dev.local/graphql  # [drawbridge]
REDIS_HOST=redis.dev.local  # [drawbridge]
FACEBOOK_APP_ID=630728900463808

[fresha-marketplace-platform]

👋 Thanks for opening this issue, @radiosilence!

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• Steps to reproduce (if it's a bug)
• Expected vs. actual behavior
• Environment details

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