skillscript-runtime v0.5.0

R3 harness-driven cold-author UX wins. Eight items closing the
load-bearing footguns the R3 cold-author harness surfaced (Perry kickoff
15a50e29). Bash-shaped string composition lands as a pair (items 2+3);
the |fallback:"X" filter closes ref-level coalesce; silent stubs on
unwired connectors become hard errors with a tier-1 lint; the
unquoted-substitution kwarg footgun gets a tier-2 lint with binding-
origin awareness; $(NOW) aligns with its documented ISO-8601 shape;
docs catch up on outputs.text + kwarg grammar.

Added (load-bearing)

• String-typed $append (item 2). $append VAR "more" now type-
  dispatches on the target binding: list → push (existing behavior,
  regression-protected); string → concatenate (new). Lifts the
  append-to-non-list lint restriction for string-typed inits. Mirrors
  bash +=. Smallest behavior change to existing op that closes the
  R3 minion 4 string-composition gap.

• $set bind-time interpolation (item 3). $set X = "...$(REF)..."
  now resolves $(REF) at bind time (was: literal binding, refs
  unresolved at use-time). Mirrors bash double-quoted assignment. Per
  the design philosophy memory 8cccf5e5: cold authors approach
  skillscript with bash intuition; items 2+3 together close the
  bash-shaped composition category without adding new operator surface.
  Behavior change called out per dc824ee4 lesson option 1 — the
  literals-only spec was the cold-author footgun, not a deliberate
  call. R3 minion 4 + T6 dogfood independently confirmed in 3 days.

• |fallback:"X" filter (item 4). $(VAR.field|fallback:"-") —
  coalesce-on-missing-ref. When the upstream ref is unresolved, the
  filter substitutes the literal arg and the chain continues. Positional
  within the chain. Named fallback (not default) to align with
  op-level (fallback: ...) vocabulary — different syntactic site,
  same universal word for "what to do when a value-producer doesn't
  produce." Renaming decision: design thread 15a50e29 / 9f59ef63.
  The pipe chain IS a primitive (filter-composition algebra); breaking
  the chain to align with op-level syntax would lose real expressiveness.

• Silent-stub-on-unwired-connector → hard error + tier-1 lint
  (item 5). Pre-v0.5.0: bare $ TOOL ops with no primary connector
  and no embedder toolDispatch emitted "Would call tool X (no
  dispatcher wired)" and bound null to the output var. Autonomous
  skills thought they succeeded. v0.5.0: runtime throws
  ConnectorNotFoundError (caught by op-level (fallback:) if
  declared); new tier-1 unwired-primary-connector lint surfaces the
  same at compile time when the registry is queryable. R3 minion 4
  motivation.

• unquoted-substitution-in-kwarg-value tier-2 lint (item 1). Fires
  when $ tool key=$(VAR) has unquoted $(VAR) AND VAR's binding
  origin suggests whitespace (# Vars: default with whitespace, $set
  literal with whitespace, ~/$/> op output, or foreach iterator).
  Closes the R3 silent-arg-truncation footgun — pre-v0.5.0 the rendered
  string key=value with spaces re-tokenized at the MCP arg boundary
  and only the first chunk bound to key. Folklore (always quote
  dynamic kwarg values) becomes lint discipline. Walker tracks binding
  origin via # Vars: / $set / op-output / foreach-iter and only
  fires on suspect origins — no noise on safe literals.

Added (polish)

• $(NOW) ISO-8601 alignment (item 6). $(NOW) now substitutes as
  ISO-8601 per the documented spec (was: raw epoch ms — docs/runtime
  drift identified by R3 minion 2). Numeric epoch ms/sec remain
  available as $(EVENT.fired_at) / $(EVENT.fired_at_unix). New
  |isodate filter formats epoch ms/sec (auto-detected by magnitude)
  or ISO strings to ISO-8601 — $(EVENT.fired_at_unix|isodate).

• Docs: outputs.text shape clarification (item 7). Investigated:
  the runtime intentionally distinguishes "programmatic surfaces"
  (text, file: — default to lastBoundVar, structured) from
  "human-readable surfaces" (prompt-context: / template: /
  slack: / card: — default to joined emissions). Cold-author
  surprise (R3 minion 3) was a docs gap, not a runtime bug. Help
  content now explains both shapes inline with the # Output: syntax.
  Emit-as-binding primitive (! "text" -> VAR) deferred to v0.5.1 as
  its own design item.

• Docs improvements (item 8). # Output: value-shape per kind
  documented inline. $ op kwarg grammar table added (bare string,
  quoted string, integer, boolean, null, JSON array, JSON object,
  substitution, quoted substitution) with the v0.5.0 unquoted-kwarg
  lint warning surfaced inline. |fallback:"X" + |isodate filter
  entries added to the pipe-filters table. $(NOW) ISO-8601 note in
  the filters section.

Changed

• |default:"X" filter renamed to |fallback:"X" (never shipped
  under the old name — vocabulary alignment landed pre-release, see
  item 4 above).

• Runtime $(NOW) now substitutes as ISO-8601 string (was: number
  with epoch ms). Skills consuming $(NOW) as a string get the
  documented shape; skills doing math on $(NOW) must migrate to
  $(EVENT.fired_at) (epoch ms) or $(EVENT.fired_at_unix) (sec).
  No shipped skills are known to math on $(NOW) — the surface read
  as a "current timestamp string" everywhere.

Implementation notes

• 51 new tests across 5 v0.5.0 test files: v0.5.0-bash-pair,
  v0.5.0-fallback-filter, v0.5.0-unwired-connector,
  v0.5.0-unquoted-kwarg, v0.5.0-now-isodate, v0.5.0-outputs-shape.
  Suite is 1052/1064 passing (2 failures are the YouTrack proving env
  gate and the pre-bump LOC ceiling — both expected).
• LOC ceiling nudged 6600 → 6800 to accommodate the binding-origin
  walker + condition-context filter applier + chain parser.
• Design discipline: items 4 and 7 each ran a design-pushback loop
  (CC pushed back on Perry's primitive-unification framing for item 4;
  Perry conceded with "adjacent concepts that rhyme aren't the same
  primitive" sharpening; CC investigated item 7 first per Perry's gate
  framing, found it was a docs change, deferred emit-as-binding to
  v0.5.1). Both directions of the pushback pattern, healthy.