LaunchOpts.Agent (string) and WorkspaceEntryOptions.Agent (string) are
replaced by *agent.Resolved, carrying Command, Args, and Env. The five
entry commands (new, resume, last, open, attach) each construct an
AgentSpec from the workspace metadata, apply --agent as a one-shot
agent.command override (Open Q 1 — keeps muscle memory for users
passing executable paths), call agent.Resolve, and pass the result
through. resolveEntryAgent centralises the resume/last/open/attach path.
shell.ExecAgent and shell.ExecTmuxAgent gain an args parameter; agent.env
is merged into the env map at the session.Run launch switch, AFTER
ctask's exported CTASK_* vars (per spec §5: agent.env wins on collision).
mergeAgentEnv is the centralised merge.
Lease, manifest, write lock, heartbeat, summary, and provisional
cleanup are unchanged. The Agent string fields on Lease, SessionSummary,
and SessionInfo continue to record the launched command for diagnostics.
Replace TaskMeta.Agent (string) with TaskMeta.Agent (AgentSpec) carrying
type/command/args/env. Custom UnmarshalYAML preserves the legacy scalar
form: a built-in name (claude, opencode) maps to that type; any other
scalar maps to type=custom with the scalar as command. A missing agent
field leaves Type empty so the resolver fills in default_agent at launch.
ValidateAgentSpec enforces: known type (claude|opencode|custom),
type=custom requires command, command must be an executable name or
path with no whitespace or shell metacharacters.
Launch-path wiring (Task 3) and the --agent flag rework (Task 4) are
intentionally not part of this commit; cmd/* call sites are patched to
the minimum needed for the build to compile.
typebasedio