refactor(run-mode): collapse the run-mode vocabulary and rewrite the mode docs

[slowdini]

Part of #113 ("Instant magic"). The user-facing tail of the in-session removal (#B). Doc-heavy.

Closes/supersedes #31 (run-mode tracking ticket): once one dispatch path remains, the multi-mode vocabulary collapses.

Why

After the in-session mechanism is removed (#B), the three-mode story (interactive / hybrid / headless) collapses to a single Cli dispatch path. "Headless" and "hybrid" were always the same mechanism differing only in who drives the loop, so we reframe headless as tacitly supported — the same runbook a human or an agent can drive — rather than a first-class mode carrying its own support claims. The --run-mode flag and RunMode enum become vestigial and the docs that narrate multiple modes need a rewrite.

Scope

• Retire --run-mode: remove the flag, or keep it as a hidden deprecated no-op for one release. Collapse the RunMode enum now that one path remains.
• Drop capabilities_for fields that existed only to gate mechanism choice.
• conditions.json: stop recording run_mode (or pin a constant). Minor back-compat change to a version-controlled artifact — add a migration note and bump the version. (Schemas in schema/ are harness-agnostic and don't validate run_mode, so no schema break.)
• README rewrite — these sections all assume the multi-mode world:
  • Quickstart (Mode A / Mode B walkthroughs reference switch-condition and "start a fresh interactive session").
  • The run loop.
  • Run modes (collapse the three-mode table → one Cli dispatch flow; reframe headless as tacit).
  • Cost and billing (the interactive-vs-claude -p billing split is moot).
  • Per-harness mode paragraphs (Claude Code "fully wired" / Codex / OpenCode).
• docs/harness-parity.md: remove the "two parity axes / dispatch-mechanism" framing (the mechanism axis is gone). (The full minimal-interface rewrite is #D.)

Files

src/cli/args.rs (--run-mode), src/core/run_mode.rs, the conditions.json writer + src/core/context.rs, README.md (large surface), docs/harness-parity.md, and affected tests.

Acceptance criteria

• No --run-mode surface (or a clearly deprecated no-op).
• README tells one coherent single-mode story; no dangling references to switch-condition / interactive sessions.
• conditions.json change documented + version bumped.
• cargo test, cargo fmt --check, cargo clippy --all-targets -- -D warnings green.

Dependencies

Depends on #B.

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Filed from the #113 feasibility/breakdown plan.