Allow operator settings.yaml to configure the default per-project memory store

Problem

Embedded mecatui enables project memory by default, but the project-memory directory can be changed only per invocation with --memory-dir. The default derives one isolated store from each exact workspace path:

text ~/.local/share/mecatui/memory/-home-jbeda-src-mecatl ~/.local/share/mecatui/memory/-home-jbeda-src-mecatl-2 ...

This makes a set of local Git worktree/agent slots for the same project receive separate project memory. There is currently no operator-owned YAML setting to configure a shared store for that set. ~/.config/mecatui/settings.yaml is client UI configuration; ~/.config/mecatl/settings.yaml is the appropriate operator-owned server configuration, but it does not currently expose this setting. A project-local .mecatl/settings.yaml should not control a host persistence path.

Requested behavior

Add an operator-tier key in /mecatl/settings.yaml that sets the default local project-memory directory (or a documented per-workspace mapping/override). It should apply only to embedded/local server composition, with explicit --memory-dir retaining higher precedence and --no-memory still disabling memory.

The immediate use case is intentionally sharing one file-backed store across several local worktrees/agent slots of the same Mecatl repository, while leaving ordinary repositories on their existing per-path default.

For example, an operator should be able to configure a single store such as:

text ~/.local/share/mecatui/memory/mecatl-slots

without maintaining a separate wrapper command solely to pass --memory-dir.

Design constraints

• Operator/global configuration only; do not accept this from project-tier settings.
• Treat the configured path as a local persistence destination, never as workspace/placement authority.
• Preserve the current default when unset: /mecatui/memory/<full-workspace-path-slug>.
• Define clear precedence: --no-memory > explicit --memory-dir > operator setting > computed default.
• Validate and report configuration errors without leaking sensitive config values.
• A mapping/pattern form may be preferable to one global directory if the design wants to avoid unintentionally merging unrelated projects; the minimal useful form can be a single operator-selected default directory.