feat: workspace.static soft override with env-var fallback

[christso]

Problem

Workspace configuration has accumulated overlapping controls across YAML and CLI, which makes behavior hard to predict.

Current confusion points:

1. Multiple ways to express static workspaces (mode/static_path, CLI path flags, legacy forms).
2. Mode and path interactions are implicit in places and strict in others.
3. Legacy pooling and retention flags add extra precedence branches.

We want a single, obvious configuration model for workspace behavior.

Proposal: canonical workspace.mode + workspace.path

Adopt one workspace shape in YAML:

workspace:
  mode: pooled | temp | static
  path: /abs/path              # required only when mode=static
  template: ./template         # for pooled/temp flows
  repos: [...]                 # for pooled/temp flows
  hooks: {...}

Semantics

• mode=pooled → use pooled workspace lifecycle.
• mode=temp → materialize fresh workspace per run (no pooling).
• mode=static → use an existing user-managed workspace at workspace.path.

Defaults and implied mode

• If no mode/path is provided, default to pooled.
• In YAML, non-empty workspace.path with missing workspace.mode implies static.
• In CLI, --workspace-path implies --workspace-mode static.

Validation rules

• If mode=static and path is missing/empty → hard error.
• If --workspace-path is set and --workspace-mode is provided with non-static value → hard error.

Precedence (highest first)

1. CLI --workspace-path (implies static)
2. CLI --workspace-mode
3. YAML workspace.path / workspace.mode
4. Default mode=pooled

CLI surface simplification

Keep only mode/path workspace controls in CLI:

• Keep --workspace-mode pooled|temp|static
• Keep --workspace-path

Remove these flags entirely (hard remove, no warnings/transition period):

• --keep-workspaces
• --cleanup-workspaces
• --retain-on-success
• --retain-on-failure
• --pool-workspaces
• --no-pool

Cleanup/retention behavior remains hook/config-driven (before_each / after_each / after_all), not CLI-driven.

Schema cleanup (hard remove)

Remove legacy/duplicate workspace fields and aliases:

• Remove workspace.static (if introduced in prior drafts)
• Remove workspace.static_path
• Remove workspace.pool boolean
• Remove target-level workspace_template

Use only eval-level workspace.template / workspace.mode / workspace.path.

Acceptance signals

• YAML supports canonical workspace.mode + workspace.path
• Default is mode=pooled when neither mode nor path is set
• YAML workspace.path without mode implies static
• CLI --workspace-path implies static without requiring --workspace-mode
• CLI rejects --workspace-path with non-static --workspace-mode
• mode=static without path is rejected
• Removed CLI flags are no longer accepted
• Removed legacy YAML/target fields are rejected (static_path, pool, target workspace_template)
• CLI help/docs reflect only mode/path workspace controls and precedence

Non-goals

• No backward-compat aliases for removed fields/flags
• No warning-based transition period
• No changes to core hook execution semantics

Related

• feat(core): support env interpolation across all config string fields #510 — env interpolation across all config string fields (merged)