feat(agents): add max_agents_per_repo config to allow multiple agents per repo

[PieterPel]

Problem

When git.use_worktrees = true, each ticket gets an isolated worktree at a different path, so there is no technical reason two agents can't work on the same repo simultaneously. However, the dispatch logic in try_launch and auto_launch calls State::is_project_busy() — a hard boolean that blocks any second agent on a repo, regardless of worktree isolation.

This makes the 1-per-repo limit feel arbitrary and frustrating when worktrees are enabled.

Proposed solution

Add a max_agents_per_repo field to [agents] in config.toml:

[agents]
max_parallel = 5
max_agents_per_repo = 3  # default: 1 (fully backward-compatible)

When set to 1 (the default), behaviour is identical to today. When set higher, the dispatch logic counts running agents for that project and only blocks when the count reaches the limit.

Implementation

• Add max_agents_per_repo: usize to AgentsConfig with #[serde(default = "default_max_agents_per_repo")] (default returns 1)
• Add State::project_agent_count(project: &str) -> usize alongside is_project_busy()
• Refactor is_project_busy() to delegate to project_agent_count() >= 1 (no behaviour change)
• Update try_launch and auto_launch to compare project_agent_count against max_agents_per_repo
• Status message updated to show N/MAX agents active for clarity

A working implementation is available in PieterPel/operator#feat/multiple-agents-per-repo.