Stale `zellij` references in comments & docs after Zellij → tmux migration (#404)

[i-trytoohard]

Stale zellij references remain after Zellij → tmux migration (#404)

PR #404 migrated the runtime adapter from Zellij to tmux (Darwin/Linux) + conpty (Windows), selected via runtimeselect. The functional backend/internal/adapters/runtime/zellij/ package is gone — but ~30 stale zellij references remain in comments and docs, describing zellij as the current runtime. This misleads readers and tooling (code reviews, AI analysis) into reporting zellij as active.

Stale code comments (describe zellij behavior as current)

File	Line	Reference
observe/reaper/reaper.go	155	"transient Zellij outage"
observe/reaper/reaper_test.go	71	errors.New("Zellij gone")
terminal/attachment.go	15	"runtime adapters (tmux/zellij/conpty)"
terminal/doc.go	2–28	entire file describes per-client zellij attach PTY model
review/launcher.go	37	"The zellij runtime..."
cli/client.go	18	"zellij launch"
ports/outbound.go	104	"tmux/zellij back it with a local PTY"
ports/agent.go	13	"empty zellij pane"
session_manager/manager.go	272	"Zellij happily creates a session+pane around a missing command"
session_manager/manager_test.go	1049	"empty zellij pane"
adapters/agent/claudecode/claudecode.go	13	"via zellij attach"
adapters/agent/qwen/qwen.go	150	"empty zellij pane"
adapters/agent/kilocode/kilocode.go	207	"matches the zellij runtime"
adapters/agent/aider/aider_test.go	286	"empty zellij pane"
adapters/agent/devin/devin_test.go	260	"empty zellij pane"
adapters/agent/goose/goose_test.go	385	"empty zellij pane"
adapters/agent/auggie/auggie_test.go	189	"empty zellij pane"
adapters/agent/kiro/kiro_test.go	390	"empty zellij pane"
adapters/runtime/conpty/attach.go	2	"tmux/zellij"
adapters/runtime/conpty/runtime.go	155	"tmux/zellij return a non-nil error"
adapters/runtime/tmux/tmux_test.go	16	"mirrors zellij_test.go" (file no longer exists)
adapters/runtime/ptyexec/spawn_unix.go	3	"attach CLI (tmux/zellij)"

Stale docs

File	Issue
docs/backend-code-structure.md	Lists internal/adapters/runtime/zellij as a package (lines 45, 49, 171, 184, 193); describes "zellij attach PTY" model (lines 242–243)
docs/STATUS.md	Line 53: "per-client zellij attach PTY"
docs/daemon-environment.md	Lines 11, 22, 30: describes daemon execing zellij, chain daemon -> zellij -> agent
docs/cli/README.md	Line 20: ao doctor checks optional zellij

Proposed fix

Mechanical find-and-replace pass:

• Update comments to say "tmux" (or "tmux/conpty") where they describe current behavior.
• Rewrite terminal/doc.go and docs/backend-code-structure.md to reflect the tmux + conpty + runtimeselect architecture.
• Update docs/STATUS.md, docs/daemon-environment.md, docs/cli/README.md.
• Test-comment references ("empty zellij pane") can stay or be updated for consistency.

No behavioral change — docs/comments only.

\cc @harshitsinghbhandari (author of #404)