Feature request: allow per-agent skills propagation instead of global --skills/--no-skills only

[456838]

Problem

Ruler currently treats skills propagation as a global switch.

From the README:

• default_agents controls which agents are applied by default
• skills are enabled by default
• skills can only be controlled globally via:
  • ruler apply --skills
  • ruler apply --no-skills
  • [skills].enabled = true/false

In practice, this makes it hard to use Ruler in mixed-agent projects.

Example:

default_agents = ["codex", "trae", "cursor"]

[agents.codex]
enabled = true

[agents.cursor]
enabled = true

[agents.trae]
enabled = true

[agents.claude]
enabled = false

Even with only codex, trae, and cursor intended for active use, skills-related output can still become confusing, and there is no explicit way to say:

propagate skills to codex
propagate skills to cursor
do NOT propagate skills to claude
do NOT propagate skills to any other supported agent
Expected behavior
Ruler should support agent-level control for skills propagation.

Something like this would solve the problem:

[skills]
enabled = true

[agents.codex.skills]
enabled = true

[agents.cursor.skills]
enabled = true

[agents.trae.skills]
enabled = false

[agents.claude.skills]
enabled = false

Or equivalently:
[skills]
enabled = true
allowed_agents = ["codex", "cursor"]

Why this matters
default_agents is not expressive enough for skills distribution.

There are real cases where users want:

rules/config for some agents
skills only for a subset of those agents
clean repositories without extra generated skill directories for tools they are not using
Without per-agent control, the only safe option is disabling skills globally, which is too coarse.

Proposed solution
Any of these would help:

Per-agent skills enable/disable
Global skills.allowed_agents = [...]
ruler apply --skills-agents codex,cursor