Post-C.6 cleanup: delete legacy linenoise REPL (~4,235 LOC)

[jsavin]

Rationale

After C.6 (default flip — frontier-cli defaults to boxen REPL) ships and a release cycle passes without users / scripts hitting issues with the --plain fallback, the legacy linenoise REPL can be removed entirely.

This is the final step of the REPL schism resolution plan (planning/phase_c/REPL_SCHISM_EXECUTION_PLAN.md Section 8). Deferred to a separate PR specifically so the boxen REPL gets real-world exposure as the default before linenoise is gone for good.

Scope

Code to delete (~4,235 LOC):

• frontier-cli/repl.c — the linenoise-driven REPL
• frontier-cli/repl_eval.c / repl_eval.h — if scoped only to legacy path (verify via grep)
• third_party/linenoise/ — bundled linenoise library
• --plain CLI flag in frontier-cli/cli_parser.c — the fallback that selects linenoise
• --debug-tui deprecation no-op shim (already a no-op alias after C.6) — fully remove

Code to clean up:

• frontier-cli/boxen_repl.c constant duplication comments (lines around BOXEN_REPL_HISTORY_FILE, BOXEN_REPL_HISTORY_SIZE) that say "MUST stay byte-identical with repl.c:85-86 until linenoise REPL is removed in milestone C.6" — remove the comments and consolidate
• frontier-cli/repl_completion.h — the adapter header introduced in C.0.2 (PR feat(repl): C.0.2 Tab completion for boxen REPL #769) to share completion logic; the boxen path can absorb the logic now
• frontier-cli/repl_slash_resolver.{c,h} and frontier-cli/repl_slash_dispatch.h — if scoped only to legacy (verify); boxen has its own slash dispatch
• frontier-cli/repl_palette_source.{c,h} — verify boxen path is the only caller; if so, fold under boxen_palette_backend or rename
• frontier-cli/palette.c constant duplication — the repl.c constants comment can drop the "byte-identical with repl.c" qualifier

Issue #773 follow-up: extract dispatch_synthesized_script to a shared TU — this becomes simpler when there's only one REPL consumer, and can be folded into this cleanup.

Tests to delete: any pre-existing test that exercises the linenoise REPL specifically (search for --plain, linenoise-specific helpers, dispatch_slash_command direct callers in repl.c).

Tests to retain: anything in tests/repl_* that exercises shared infrastructure (slash resolver, palette source, completion logic) — keep but verify they still pass against the boxen-only build.

Manifest update:

• CLAUDE.md ## /auto Test Manifest — remove any linenoise-specific notes

Constraints

• DO NOT execute until C.6 has soaked. "Soaked" means at least one user-driven release cycle where the new default has been in use and no --plain fallback complaints have surfaced.
• DO NOT delete in this issue's PR: frontier-cli/boxen_repl.c, frontier-cli/boxen_palette_backend.c, frontier-cli/boxen_completion_popup.c — those are the new REPL.
• DO NOT delete: third_party/termbox2/ — boxen depends on it.

Exit Criteria

• make -C frontier-cli clean
• ./tools/run_headless_tests.sh all green
• cd tests && make test-integration matches baseline character-for-character
• ./tools/tui-tests/run.sh all green (TUI parity verified)
• --plain flag removed; --debug-tui shim removed; only frontier-cli (no flag) launches the REPL
• Release notes draft entry: "Removed legacy linenoise REPL and --plain fallback. The boxen REPL is now the only REPL."
• LOC reduction verified (~4,235 LOC removed; track in PR description)

Context

• Sequence: C.6 (default flip — PR pending) → release cycle → this issue
• Refs: Interactive REPL debugging: provide a debug transport for bare linenoise sessions #691 (umbrella), C.0.x: end-to-end behavioral test for DONE_EXECUTE -> close -> dispatch chain #773 (dispatch_synthesized_script extraction follow-up)
• Planning: planning/phase_c/REPL_SCHISM_EXECUTION_PLAN.md Section 8
• Severity: P1 because it's load-bearing for closing out the schism resolution and removing the migration-window duplication comments scattered through the boxen code. Not urgent (waits on C.6 soak), but high-value when ready.