refactor(shell-plugin): rename _FORGE_TERM_ENABLED to _FORGE_TERM

[tusharmath]

Summary

Rename the internal shell-plugin variable _FORGE_TERM_ENABLED to _FORGE_TERM and its corresponding user-facing environment variable FORGE_TERM_ENABLED to FORGE_TERM for a shorter, more consistent naming convention.

Context

The variable _FORGE_TERM_ENABLED served as the master switch for terminal context capture (preexec/precmd hooks). The _ENABLED suffix was redundant — the variable's boolean value already communicates enablement state. Dropping it aligns with the naming style of other _FORGE_TERM_* variables in the plugin (e.g., _FORGE_TERM_MAX_COMMANDS, _FORGE_TERM_COMMANDS) and reduces visual noise in the shell config.

Changes

• Renamed _FORGE_TERM_ENABLED → _FORGE_TERM across all shell plugin files
• Renamed the user-facing env var FORGE_TERM_ENABLED → FORGE_TERM in config.zsh
• Updated all reference sites in config.zsh, context.zsh, and helpers.zsh

Key Implementation Details

The rename is purely mechanical — no logic, defaults, or behavior changed. The variable still defaults to "true" and controls registration of the preexec/precmd hooks as well as terminal command context forwarding to the forge process.

Testing

# Source the plugin and verify terminal context capture still works
source shell-plugin/forge.plugin.zsh

# Confirm the new variable name is respected
export FORGE_TERM=false
# Run a command — context should NOT be captured

export FORGE_TERM=true
# Run a command — context SHOULD be captured