feat: per-workstream environment variable overrides

[davidpoblador]

Problem

Currently, all workstreams in a project share the same environment variables (the global FF_* vars plus whatever's in the symlinked .env). There's no way to set workstream-specific overrides — for example, a different DATABASE_URL per branch, or a custom port override.

FF_PORT already provides a unique deterministic port per worktree (range 40001-49999), which solves port conflicts for run scripts. But users may need other per-workstream differences (database URLs, API keys for staging environments, feature flags, etc.).

Proposal

Add a key-value editor in the workstream or project settings where users can define environment variable overrides per workstream. These overrides would be:

• Injected into all terminal sessions for that workstream (agent, setup, run, regular terminals)
• Available alongside the existing FF_* variables
• Higher priority than .env symlinked values

Use cases

• Different DATABASE_URL per branch
• Custom feature flags per workstream
• API endpoint overrides for staging/preview environments

Context

Raised in #329 — the setup/run scripts and port detection parts of that request are already implemented via .factoryfloor.json and FF_PORT.