Use current zellij session instead of hardcoded zelma-main

[dapi]

Problem

zelma instances create currently falls back to the hardcoded zellij session name zelma-main when ZELMA_ZELLIJ_SESSION is not set. This makes the CLI target a fixed session instead of the zellij session where the user is currently working.

Observed touchpoints:

• internal/create/create.go: DefaultZellijSession = "zelma-main"
• internal/cli/cli.go: configuredZellijSession() returns create.DefaultZellijSession
• memory-bank/ops/config.md: documents ZELMA_ZELLIJ_SESSION default as zelma-main
• tests and e2e fixtures currently assume zelma-main

Desired Behavior

When ZELMA_ZELLIJ_SESSION is set, keep honoring it as an explicit override.

When ZELMA_ZELLIJ_SESSION is not set, resolve the current attached zellij session and use that as the target for instances create / pane launch workflows. zelma-main should not be the production fallback.

Suggested Design Notes

• Add a small zellij adapter capability to discover the current zellij session from the running environment or zellij CLI, with deterministic diagnostics when unavailable.
• Keep tests free to use fixture names like zelma-main, but avoid treating that name as a production default.
• Update help text, config docs and recovery hints so the default is described as “current zellij session”, not zelma-main.
• Preserve explicit ZELMA_ZELLIJ_SESSION behavior for scripts and Docker e2e.

Acceptance Criteria

• instances create without ZELMA_ZELLIJ_SESSION targets the current zellij session.
• instances create with ZELMA_ZELLIJ_SESSION=<name> still targets <name>.
• No production code path hardcodes zelma-main as the default target session.
• If current session resolution is impossible, CLI returns an agent-friendly diagnostic and does not launch a pane or write registry state.
• README/help/config docs describe the new resolution order.
• Unit tests cover current-session resolution, env override precedence and failure diagnostics.
• E2E fixtures may still use zelma-main as test data, but tests assert behavior rather than relying on a global production default.