docs: name worker sessions with an imperative sentence

[LeTuR]

What

Renames the convention for thurbox worker session names — no existing session
is renamed. Names go from kebab-case slugs to an imperative summary in sentence
case:

Run exec automations off the TUI thread     good
Document the customization surface          good
thurbox-automation-nonblocking              bad — a kebab slug, reads as an id
Fix stuff                                   bad — says nothing

Documentation only. No script or behavior changes.

Why

A session name is the first thing an operator reads in the TUI window list and
the run log. fleet-ship-<slug> reads as an identifier for the work; Cache the registry sync between runs reads as the work. The repo prefix was pure waste —
the session already carries the repo in its cwd, and the run log has a column
for it.

Spaces are verified to round-trip. A throwaway session confirmed that
session create --name 'Probe sentence casing' succeeds, session get --json
returns "name": "Probe sentence casing" intact, and
message send --to 'Probe sentence casing' returns {"enqueued":true,"woke":true}.
The playbook template's "hyphenated, no spaces" rule was therefore wrong, not
merely stylistic. The real constraints on --name are 1–64 chars, no slashes,
no leading ..

Because the name is a mailbox address, the --to / --for examples are now
quoted: an unquoted spaced name is split by the shell and mails somewhere else.
The branch stays kebab-case and keeps its slashes (--worktree-branch
fix/automation-exec-nonblocking) — the branch is not the name.

Changed

• .claude/skills/thurbox-session/SKILL.md — new Naming a session
  subsection under §1; --name flag row corrected; session create examples
  (single- and multi-repo) now demonstrate the convention; mailbox addresses
  quoted.
• orchestration/runs/_TEMPLATE.md — sentence-case example in the session table
  and timeline.
• orchestration/playbooks/{_TEMPLATE,ship-feature,cross-repo-sweep}.md — name
  guidance updated. cross-repo-sweep keeps the repo in the name (as the
  object of the sentence, bare, no owner — --name takes no slashes), since a
  sweep runs one goal N times and identical mailbox addresses are ambiguous.
• README.md — the "seventh appearance of fleet" paragraph described the old
  fleet-<slug> convention; reworded. Mailbox examples quoted.

The fleet session, agent, and extension are untouched — that name is the
deliberate six-place coupling the README's "Customizing" section describes, and
it is not what this PR is about.

Gates

shellcheck scripts/*.sh passes; git diff --stat shows .md files only.