caos-cli: remove /pr, /publish-branch and the client publisher

[nishu-builder]

Third of three, stacked on #301 (std/github), which sits on #300 (the design). Net about −2,400 lines.

The agent publishes now: publish_source pushes the commit from the server's store, and caos-std/github opens or updates the PR. This removes the client's own route to the same end. With it goes the client's need for gh, a host-side git push, and a local copy of the history it publishes.

Removed

• TUI: /pr and /publish-branch; the preview modal and its key and mouse handling; the publishing state and its status, sidebar and footer rendering; and the PublicationPlanned, Published and PublicationBaseImported messages (bin/tui/publication.rs).
• src/publication.rs: gh pr create / gh pr edit.
• source_trees.rs: the publication half: preview, provenance inference, base import, default_branch, branch_snapshot.
• host_git.rs: the PR half.
• lib.rs: the client push (publish_source_tree_branch and friends), reject_publish_caos, publication_diagnostic, PublishedBranch, and append_system_notice, which only the publisher called.
• Their tests.

Kept

• Publication records stay readable. The agent's publish_source still writes them, and conversation_load still summarizes them. A new test appends one the way llm-step does and loads it back.
• Two escape-key tests lose their publishing cases and keep the running ones.

Docs

• chat.md's "Publishing with /pr" becomes a short "Publishing" section pointing at publish_source and caos-std/github.
• TUI.md, SPEC.md ("Publication"), README.md and agent-harness.md drop the commands.
• Following the review on design: PRs through a std/github tool, not gh or /pr #300, SPEC.md and TUI.md no longer say a .caos entry blocks publication: nothing checks for it at publish time.

Testing

• cargo test -p caos-cli: 39/39 lib tests and 148/150 bin tests pass. The two failures are plain_commit_fork_failure_never_becomes_a_markerless_conversation and checkout_import_completes_partial_history_and_keeps_local_edits. They fail the same way on std/github: interact with PRs and stacks through github api #301's head, without this change, on this machine's git 2.43: one on a promisor lazy fetch, one on a git error message.
• cargo clippy -p caos-cli --all-targets -- -D warnings and cargo fmt --all --check are clean.
• Not run: the caos suite and nix build. They need flake inputs from GitHub repos this session couldn't reach. The release workflow's nix build .#caos-cli on push covers the client build.

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https://claude.ai/code/session_01CZGkTta5NYxhXcLPZwH3q1