feat(config): expose FORGE_* env vars via :config-env command

[marchantdev]

Fixes #2638

Problem

Many Forge tuning knobs — retry limits, HTTP timeouts, tool timeouts, max file sizes — are only discoverable by reading the source code or documentation. Changing them requires manually editing a .env file. There is no in-shell way to list what is configurable or to set a value without leaving the terminal.

Solution

This PR adds a :config-env (:ce) command and the corresponding CLI plumbing so users can browse, inspect, and set all FORGE_* environment variables without touching a file.

CLI changes

Command	Behaviour
forge config get env	Lists all 38 known FORGE_* vars with current values and defaults
forge config get env KEY	Shows the current value of a single variable
forge config set env KEY VALUE	Writes KEY=VALUE to .env in cwd and applies immediately (no restart needed)

ZSH changes

• :config-env / :ce — opens an fzf picker populated by forge config get env; selecting a variable prompts for a value and calls forge config set env automatically
• Registered in built_in_commands.json so it appears in :help and tab completion

Example

# List all configurable variables with current values
$ forge config get env
FORGE_TOOL_TIMEOUT              300  (default)   # Max seconds a shell tool is allowed to run
FORGE_MAX_SEARCH_RESULT_BYTES   10240  (default)  # Max bytes returned by a file-search result
...

# Get a specific variable
$ forge config get env FORGE_TOOL_TIMEOUT
FORGE_TOOL_TIMEOUT (not set, default: 300)

# Set a variable (writes to .env, applies immediately)
$ forge config set env FORGE_TOOL_TIMEOUT 60
● FORGE_TOOL_TIMEOUT  is now set to '60' in /home/user/myproject/.env

# Or interactively via ZSH
:ce   # Opens fzf picker → select variable → enter value

Variables exposed

All 38 FORGE_* variables including: tool timeouts, file size limits, HTTP/TLS config, retry settings, semantic search parameters, API URLs, conversation history settings, and dump/debug options.