sl0p: per-pane workspace hints (purpose=)

Implements the zellij half of sl0p/workspace-hints-proposal.md: a pane can carry
an opaque agent-facing hint for what it is FOR, so a project can ship a layout
that an agent is able to navigate instead of guessing.

zellij never interprets a purpose. The consuming agent owns the entire vocabulary,
which is what keeps this to one string threaded along the existing status_bar path
rather than a policy engine, and lets the vocabulary change without a rebuild.

Two ways in, because declared panes and agent-created panes are different:
  layout:  pane name="server" purpose="dev-server"
  in-band: printf '\033]5577;1;purpose;agent:build\033\\'
The second exists because tagging at spawn time would mean threading a field
through 28 SpawnTerminal call sites, and a new action variant is impossible while
protobuf enums cannot be regenerated. Wrapping the command costs one printf.

A declared purpose outranks an in-band one and cannot be overridden or cleared by
it. OSC arrives from whatever the pane happens to be showing, so otherwise a
`cat hostile.txt` in the human's editor pane could relabel it as the agent's
scratch pane and invite an agent to run commands where the human is working. A
layout is written by a human in a file they can read; in-band data only fills a
blank.

Exposed on PaneListEntry (`list-panes --json`) rather than PaneInfo, which crosses
the plugin protobuf API we cannot regenerate; resolved in the screen thread where
panes live rather than via route.rs's per-pane PTY round-trips. Values are
sanitised to a bounded [A-Za-z0-9_.:/-] token at both entry points, because a
purpose is never rendered -- it is matched against and pasted into consumers'
shell and JSON pipelines.

Verified end to end rather than by inspection: purposes from a layout appear in
list-panes --json against the right pane ids, an agent-spawned pane tags itself,
a declared purpose survives an in-band override attempt, all of it comes back
after kill + resurrect, an unknown pane property is still a parse error, and a
layout with no purpose leaves dump-layout unchanged.

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